Not so hard, but coded. U can learn your questions trying the mods.
Each upgrade should be assigned to each type of unit. So Explorer as unit type has no assigned XP upgrades.

You talking about promotions? I'm talking about having XPs, so they can get promotions... Amusing how so many people read half of what others say.

Hah =)
Seems u already done your own mod with XP promotion changes.
Well, so long.

Indeed, and the issue is that the Explorer don't accumulate any XP, so he can't get the promotions I set... Too bad, that would have been nice to have veterans explorer with stealth and move into any terrains.