Both the South and the Union get two stacks of leaders in Jan '62 and '63. If you select any stack in Richmond --or Washington as the case may be-- when they appear and then look at the tool-tips on each of these stack's tab you will note that the -Division Generals- stack has only 3-1-1 leaders, even the Maj.Gen.s., and the "'62 Pool Generals" will have leaders with special abilities and/or variations in their values to the greatest extent. Other than that, there is no meaning to in which stack they were spawned.
Infantry and Cavalry brigades may be
combined with exactly one leader unit. Aside from the fact that such brigades have a leader providing them with Command Points (CP's), there is a small advantage to them having their own-personal-Jesu.... eh, leader

--in strength I think--, and for sure in survivability in battle.
A division --known internally in the game engine as a
combi-unit-- but only as "division" in the game interface and manuals and everywhere else, is a special kind of unit in that it must contain
exactly one leader and a minimum of one other unit, generally a brigade. The restrictions on what may be contained in a division are:
- Only one leader.
- No more than 14 non-leader units. A division of a leader and 17 cavalry units is for example not possible unless at lease one of those cavalry units is a brigade.
- No more than 18 sub-units (elements), including the leader
- No supply units allowed (you can actually include Field Hospitals, Signal Corpse, Engineers and Pontooneers though), but these are better left loosely in the stack, especially a corps stack.
Although you cannot combine more than one leader into a division, you can still
stack more than one leader
with a division. This is that SOP for division outside of corpse, drop a second leader into the division's stack to provide another 2 CP and your division will be fully supplied with CP's and not suffer for being under-commanded.
Since nearly every leader that ever went on to command a division, and perhaps a corps and army later, at one time commanded a brigade, and to put some flavor into the game, many brigades are named for leaders who are also provided as a unit (counter) in the game. The names of these units has absolutely no other meaning; it's just decoration.