This is a simple guide with facts regarding staff management in MGSV: TPP. In each section both the offline and online components will be seperated, so those who do not wish to participate in FOB's can simply ignore the online component.

If you could care less about how staff actually operate, just jump to Section 4 for how to acquire the best staff possible.

Some basic but important observations that I've made:

• FOB's will be essential to acquiring new soldiers with above S rank baseline stats (with out medals or other boosts)
  This means that you will never find S+ soldiers in the field with your int scope baring some freak occurrence (see more in section 3, but most likely the S+ soldiers you find in the field are boasters), no, not even if you have over a million heroism and 4 FOB's.
  *Not including the special S+ and S++ soldiers which you will recieve for finishing Act 1 and Act 2, but those soldiers are unique and not replicable.
  
• FOB's will be essential to acquiring 1 of the 3 medals and the fastest way to acquire all 3
  
• FOB's can speed up spawning and acquiring higher ranked staff at higher rates
  (see more in section 3) But most importantly.
  
• Staff levels don't actually matter that much
  Why? Because Snake (or your custom avatar) are the only ones allowed to use the bionic arm and the bandana / infinity bandana, and with all boinic arm upgrades (Active Sonar, Mobility, Precision, Medical) Snake will be the fastest moving character in the game with higher health regen and reloading than the staff you can acquire.
  Mother base wise, it is possible to research every item in the game though R&D by purely upgrading motherbase and acquiring normal S or A ranked soldiers with out bothering with any of the additional FOB features. The buffs you can gain through the base development, support, intel, and medical teams are so minute that you may not even notice them in game.
  *Note: Grade 7 weapons have been stealth patched into the game, the maximum requirement of which requires R&D team level to be at 71, this may not be possible to reach with out FOB development.

So why do I even bother getting higher ranked staff?
Well obviously to:
Achieve a level even above Big Boss!
Play as a woman.
Major soldier architypes actually have unique voice acting, including the voice commands to your buddies.
Have your soldier helmet absorb a fatal sniper shot to the head (yes this actually happens!)
and for some very minor buffs.

Offline

• Deploying a combat unit soldier with Combat rank above A++ will allow you to take more damage before dying from lethal attacks than snake, non-lethal attacks however seem to operate the exact same way, or the differences are so minute that it's not worth mentioning. This is also why hackers who have infinite health in FOB's can still be stunned and fultoned even when they are cheating.
  
• Deploying a combat unit soldier with a specific skill such as Botanist (acquire more herbs), quick draw, and so on can give you marginal benefits.
  
• Deploying with a soldier with higher ranks in other fields such as support or medical will allow you to fulton faster than soldiers with lower ranks along with other very marginal benefits (mostly nullified by the bionic arm upgrade)
  
• Marginal benefits to resource refining (faster), red circles on the map marked by intel team are smaller and more precise, faster supply drops, higher % of fulton recovering injured enemies and through sandstorms, etc.

Online

• Higher ranked soldiers defending your base will have better eye sight (spots invaders faster) as well as boosts to health and accuracy.
  
• Invading with an S rank or higher will give you marginal benefits.
  Combat: More damage resistance.
  R&D: More ammo dropped from enemies.
  Support: Faster fultoning of enemies
  Base Dev: All devices marked on first platform.
  Intel: Faster marking through int-scope.
  Medical: Longer duration of drugs (noctocyanin being the most important)
  Again mostly nullified by the absence of the bionic arm. An S++ in combat will be noticeably tougher than snake though.

With that being said let's get started.

First let's look at how we rank a staff, this is done through the ability points they have in each area, the skill that they have, their looks and gender, their morale, and their medals. I will also cover the sick bay and brig in this section.

• What happens when I acquire a staff
  Fulton: Staff goes into the brig, unfortunately nothing seems to speed up how fast they get released from the brig, heroism is a speculation but I've found no evidence of it.
  Significant injury / most prisoners: Staff goes into the sickbay, higher medical team level should speed up how fast they get released but I've not noticed it.
  Volunteers: Available immediately.
  The game then automatically sorts the new staff member into the best possible team in a queue like fashion (Team -> Waiting room -> Dismiss (not hired)).
  If the team is full and the staff member is better than someone with in it, he basically pushes the worst member off the team, that worst member goes into the waiting room, if he is worse than everyone in the waiting room he is dismissed. In the case of new members they will show up as simply "not hired".
  
  
• Sickbay / Brig
  Staff get assigned a bar indicating the length of time they have to spend in the sickbay if injured / ill / ptsd, or when extracted from fulton (volunteers are available immediately). When this bar ticks down to zero the staff gets assigned to the best possible team -> waiting room -> dismissed (not hired) in similar fashion. Not that the FOB brig operates on what appears to be daily intervals (to allow the defender to get the staff member back) so fulton extracted staff from FOB's take a long ass time to actually become available.
  
  
• Direct Contract
  When you put a staff under direct contract, you can never lose them. Not even if you deploy as them in a FOB and get killed or extracted by the defender. They are also excluded from deployments and the auto assign feature, this makes it useful to for instance keep specific staff such as Miller in a team to boost morale, keep staff in the waiting room, or keep female soldiers and the staff you are deploying to boost medals with in the combat team. You should always put your highest ranked staff under direct contract to avoid losing them from FOB infiltrations.
  
  
• Ability points (AP).
  Non-story criitical staff or unique staff (such as Hideo) are generated with a set of points in each area (see section 3 for how they are generated). If these points exceed a certain threshold the staff gets assigned that rank in that field. Unfortunately the game does not present us with a clear number next to each of the bars but with some data mining the numbers can be found:
  E: 1-15
  D: 16-31
  C: 32-47
  B: 48-63
  A: 64-79
  A+: 80-109
  A++: 110-184
  S: 185-289
  S+: 290-447
  S++: 448+
  It is possible to raise a staff's bar to exceed the upper limit of the display, it is unclear whether there is a hardcap to staff's ability points in this respect. This number is then translated onto another table which tells you how much that ability point translates into boosting the team's "Unit function" that the staff is assigned to. Your team's total level is determined by adding up all the Unit function levels of the staff that's assigned to it
  
  
• Skills
  Skills that are not story or R&D mission specific that your soldiers can gain are:
  Diplomat (lessens effect of trouble makers)
  Troublemaker (Harassment/Unsanitary/Violence)
  *this is bad and leads to decreased moral and fights breaking out (staff being sent to the sick bay), but it can be mitigated if you put 2 troublemakers in the same team. The game explains that this makes them focus on each other rather than other people.
  
  Medical specific (staff will only be assigned these if their medical ability is the highest)
  Counselor (hasten recovery of PTSD if assigned to medical)
  Physician (hasten recovery of Illnesses if assigned to medical)
  Surgeon (hasten recovery of injuries if assigned to medical)
  
  R&D specific
  Various Gunsmiths (unlocks development if assigned to R&D)
  
  Combat specific
  Quick Draw (Faster aiming)
  Botanist (acquire more herbs when picking in open world)
  Boaster (disguises rank in FOB)
  Savage (CQC while sprinting does a higher damage punch)
  Fortunate (fewever serious injuries)
  Tough Guy (higher damage resistance or more health)
  Quick Reload
  Rescuer (higher fulton recovery rate, mostly useless with high medical team)
  Athlete (faster sprinting)
  Climber (faster crawling and climbing speed)
  Gunman (more reflex time)
  
  Gunman, Botanist, Quick Draw, Athlete, Climber, and Tough Guy are the most useful skills to have.
  
  
• Cosmetics
  The soldier's look and code name is randomly generated baring the unique characters. The only way to acquire female soldiers is through rescuing them in specific missions (such as missions 14 and 23, and side-op prisoner extraction 2 and 6) as prisoners or recieving them as volunteers (barring finding them in other player's FOB's).
  
  
• Morale
  *Shoutouts to GAME_PRO24X on reddit for figuring this out
  Again the game doesn't actually display a number next to the soldier's morale bar, but morale points have the following effect:
  0: 4% chance to leave mother base after a mission / return to ACC / return to mother base trigger
  1: 2% chance to leave mother base
  2: 1% chance to leave mother base
  3: nothing
  4: nothing
  5: nothing
  6: nothing
  7. nothing
  8: +2 to all AP
  9: AP + 4
  10: AP +6
  11: AP +8
  12: AP +10
  13: AP +15
  14: AP +20
  15: AP +31
  Your soldiers join mother base with a 4-6 morale rating. You raise morale by visiting mother base and finding the soldier to salute (CQCing or knocing out soldiers when they ask for a mock battle do not actually increase morale sadly), or placing miller in the team. You lose morale by being red in GMP (that's the only way morale decreases at least according to what I've found*).
  
  *Note on Morale loss due to staff deaths on dispatch missions and troublemakers
  It's possible that staff morale decreases due to casualties in deployment missions and other parameters, but it's also possible that the message "staff morale declined" simply coincides with the deployment mission message due to other causes such as troublemakers or unseen parameters. The reason being that you only ever get updated with messages when you hit a game save point. so take that with a grain of salt.
  
  
• Medals
  There are 3 medals:
  Service Cross
  Any puppet soldier extracted from the field (includes skulls encounters), as well as wandering mother base soldiers.
  Any mother base member rescued during missions 17, 22, 27.
  Any soldiers extracted from FOB and subsequently recovered.
  
  Distinguished Service Medal
  Earn 9 Distinction points by:
  Completing missions while deployed (1-3 depending on mission rank)
  Complete displatch Mission (2 points)
  Win an infiltration mission (3 points)
  Win a FOB defense (2 points)
  Win a practice FOB defense as part of security team (1 point)
  
  Medal of Honor
  Killing an infiltrator during an FOB defense mission (must be deployed)
  
  Each medal boosts all AP by 30.

As an example, here's one of my staff members under direct contract, with all 3 medals (Left to right Medal of Honor, Service Cross, Distinguished Service Medal), who is healthy, has a decently high morale, has the skill Tough Guy, who originally had an S in combat but has been boosted to S+ (the yellow parts of the bar are bonuses).
And we also see information regarding the next threshold in the team's level to improve functionality.
So we want soldiers with high AP, good skills, good morale, good looks and code name, and all the medals, then we want to put them under direct contract to avoid losing them.

so how does the game actually handle spawning enemy soldiers?
So you've probably noticed that scanning every person in the field with your int-scope is not very efficient, and that better soldiers seem to pop up as the game advances (along with better equipment, vehicles, mines, helicopters, and so forth). After a bit of data mining it seems that it's solely based on the total number of your mother base teams, an example of this is shown here:

*Shoutouts to SardaHD on reddit for figuring this out
TppMotherBaseManagement.RegisterStaffBaseRankRange{totalSectionLv=280,g=0,f=0,e=5,d=10,c=20,b=40,a=20,s=5,sp=0,spp=0}
This means that for an average department level of 40 (total 280), the spawn rate of soldiers with atleast 1 s rank is 5%, a rank is 20%, b rank is 40, c rank is 20, d rank is 10, e rank is 5). 280 also appears to be the threshold for spawning s ranked soldiers, so as soon as you hit 280 total team level in motherbase, s ranked soldiers should start appearing.

A reasonable threshold for purely offline players is roughly between the following two thresholds
TppMotherBaseManagement.RegisterStaffBaseRankRange{totalSectionLv=385,g=0,f=0,e=3,d=6,c=11,b=30,a=35,s=15,sp=0,spp=0}
TppMotherBaseManagement.RegisterStaffBaseRankRange{totalSectionLv=441,g=0,f=0,e=3,d=6,c=11,b=15,a=35,s=30,sp=0,spp=0}
or 15% at 383 and 30% at 441. Note that I've not done the hard math on verifying whether it's possible to achieve 441 total team rating with out building any additional FOB platforms.

Does this mean that building more FOB's will spawn higher rated soldiers?
The answer seems to be no, at least not from what I can gather.
TppMotherBaseManagement.RegisterStaffBaseRankRange{totalSectionLv=686,g=0,f=0,e=3,d=6,c=11,b=15,a=35,s=30,sp=0,spp=0}
We notice that even for bases with 98 average team level, there are still zero s+ and s++ rank spawns, and the rate for s ranked soldiers is still 30%.

This isn't 100% confirmed, but as you naturally level up your staff (and especially your R&D team), your enemies recieve better ranks and equipment even during the earlier levels if you keep doing the open world activities and building mother base with out advancing the plot.

So where does heroism tie in? Heroism seems to be tied into the ranks of volunteers which will join you after you complete a mission or FOB activity, unfortunately I've yet to find a table which details this but it seems to be supported by anecdotal experience. Notice however that mission ranks, side op %, and completion % seem to have zero effect on the rank of soldiers which you will find.

But hold on a minute... If s+ and s++ ranked soldiers never appear in the field, how am I supposed to recruit them? We've already covered in the previous section that you can boost staff from s to s+ using medals and morale, but unfortunately it seems that the only way to recieve s+ and s++ soldiers is through successful FOB infiltrations and defenses and finding volunteers.

*Note: As of 1.008 it seems that boasters in the field can now appear as S+ ranked soldiers if they spawn as rank S, don't get your hopes up guys they are still just S ranked soldiers.

So we've gone over all the background information that no one cares about, how do I actually go about acquiring the highest number of the best soldiers?

Offline players
IF you do not want to touch FOB's what so ever, do the following:

• Fully develop mother base.
  
• Constantly auto assign and place the highest ranked soldiers under direct contract.
  
• Find the First Aid Manual in Bwala ya Masa and develop the Tactical (Fulton) equipment for D-Dog.
  
• Go to mission 48 [EXTREME] CODE TALKER with stealth camo, a high damage primary (Brennan works fine), and a high magazine count stun secondary or primary (riot smg, Uragan, S1000 Air-shotty, non-lethal grenades and grenade launchers). It's also been suggested that the normal version of Code talker yields the same amount of high ranking soldiers but I don't have enough data to back that up.
  
• Activate stealth camo once encountered the skulls, kill 3 of them and ignore the last one by running to code talker.
  
• Trigger the cutscene. After it ends checkpoint right away so you can run. The game will spawn large amounts of puppets to the north of the mansion, the last remaining skull may cover the southern or northern approach, do not kill the skull yet as doing so will revert the soldiers back to non-puppet status.
  
• Just leave code talker alone, the puppet soldiers pose no harm to him. Go out and just nonlethal every puppet with the camo on and spam command D-Dog to fulton them for max efficiency.
  
• Kill the last skull if necessary and evacuate. Note that you will not recieve any of the soldiers until you evac and end the mission.

This automatically gives all your extracted soldiers the service cross.
I don't recommend farming mission 22 as it is random and unpredictable, even if you spend half an hour fidgetting with direct contracts, shifting all the staff out of R&D etc. It's simply not efficient. If you do find very specificly godly staff you may want to attempt doing the rescue missions but I've yet to understand how the game picks team members. It could be possible that direct contract staff can not be taken hostage either so bear that in mind.

Online players
Fully develop mother base and develop FOB's as much as possible.
Go to relationships and support as many high platform count players as possible, preferably ones with nuclear weapons as they are likely targets.
Defend whenever possible using the best ranked soldiers with out a medal of honor and always go lethal, this nets you the medal of honor if you kill the infiltrator.
Retaliate whenever possible, retaliations have a bonus to the number of volunteers (20 for 4/4 command platform compared with the 11 or 12 for infiltrations) and are basically twice as efficient as infiltrating. Saving a staff member also gives him the service medal.
Infiltrate only when there is a volunteer bonus, don't bother infiltrating 1/4 platforms as they usually only yield S ranked volunteers at best even with perfect stealth.
Always infiltrate with a staff with 2.00 multiplier and focus on stealthing your way to the last platform, you can kill or fulton as many of the staff on the last platform as you want in anticipation of a defending player.
In the event of defending players while not even close to the objective, do whatever you want! If you manage to get the other guy to retaliate it means you have potential for further retaliations or defenses which are both great for finding more volunteers.

To obtain the distinguished service medal.

• A. Send them on deployment missions, pick the missions which specify an additional team (aside from the combat team) for maximum efficiency. Note that direct contract staff will not join deployments as there is a chance that they will die. For example, here I'm deploying 10 S ranked combat team members and 5 A++ intel team members.
  
• B. Assign specificly high ranking staff to the combat team and deploy as them to C2W and just pick the red area, use the gattling gun to destroy all 3 targets asap and the helicopter will not land and the mission ends with a quick S for 3 distinction points. Do this 3 times for a medal.

Getting rid of those pesky plot or functionality important staff with low ranks
Just put them under direct contract and put them in the waiting room, you will never lose them and you actually retain their functionality even though the game tells you to place them on the specific unit. This works for interpreters, the master gunsmith, and so on. R&D specific staff however have to be placed on the team for the development to start but after wards feel free to dismiss them altogether if they are dragging down your team level.

I hope you've found this guide helpful and again thanks to the mgs community for figuring alot of this out. There are obviously more nuances I've not covered and I will update this guide with more detailed information should I come across them. Now go build that private army of yours!

Also just for fun I will post and update this guide with the best staff member I've found so far. Let's see who can catch the shiny s++!