A interesting value regarding the cargo supply of towns, is the relative total cargo supply. It tells in % how many of the requested cargo has been delivered.
From my experience, I think it's really hard to reach 100% as towns grow bigger with supply and request more.

The reachability values tell, how many destinations (Commercial+Industrial) are available for persons. There are 3 types: Walking, Driving (Car) and using the players Lines.
In the person tab, there are minimum, maximum and average values of the selected persons.
Towns also have this value. For towns, it is the average of all people living in this town. In the town tab you see also a % value, related to all destinations. If the transport network is good and the map is not too big, this can get near 100%.
Also, the reachability values are somehow corresponding with the game's simulation performance. The number of persons is a main factor but with higher reachability, the calculation increases more.