
"standard group of guys", "any unit in a region with multiple units"?!? After so many years in this forum I don't know why I don't have a [baffled] smiley yet

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Straight Arrow, pay attention, this is basically a board game, a damn fine board game, but a board game none the less, and as such it works mechanically just like any other board game too. I imagine you've played co-sim board games (war games) before where you took two or more counters and placed them one on top of the other(s) building a stack and then placed it somewhere on the map. That is the same way you build a stack in CW2.
In CW2 you should view each unit as a counter--it is the smallest component of the game which can exist by itself on the map. If you click on a unit (counter) on the map the Stack Panel will open at the bottom of the map. Here you will see at least one unit arrayed from left to right. This is a stack, even if it is only one unit, but there can be any number of units in a stack.
So that you understand about Bragg and his division and using his ability you have to understand divisions as combi-units, but more to that in a minute.
Some units can be combined together, generally requiring that one of these units is a leader. There are two types of combined units, a leader and brigade unit, and a division, which is a leader combined with one or more other units, none of which may also be a leader.
Nearly all brigades--a brigade is a unit consisting of 2 or more elements (sub-units)--can have a leader unit combined into them.
To do this put the brigade and leader into the same stack. They do not have to be alone in the stack. Click on their stack on the map and select both units--to select multiple units, select the first unit and then control-click the second unit, so that both are highlighted--. With both the leader and the brigade selected you can now press <Cnlt><c> to combine them together; this is the simples method.
You can also use the Special Orders (SO's) "Combine Units" button to combine them. Once you have a stack opened in the Stack Panel and selected the leader and brigade as above, Click the tent at the top left of the Stack Panel to open its sub-menu, and then click on the Combine Units SO button; it looks like two soldiers with a '+' between them.
If the units you have selected are not allowed to be combined with a each other, the "Combine Units" SO button will be grayed out when they are selected and no other units are selected.
To un-combine them, select the leader and brigade combined-unit without having any other units selected and press <Cntl><d> or open the tent SO sub-menu and press "Split Units" SO button; it looks like two soldiers with a '-' between them.
Divisions are special combined units you may create starting with the Early, October '61 turn. A division my contain at the most one leader and up to 14 other units with a maximum total of 18 elements altogether, including the leader.
To create a division you must first have a leader which is activated--the envelope on his counter on the map is white:
Again, click the tent SO button to open its sup-menu. Select the leader and then click the "Create Division Command" button with the "XX" on it.
Now you can combine units with the division commander as described above.
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So, now to definitively answer your question, the Training Officer ability says that the leader with the ability must command the stack he is in for the ability to be used. If another leader is in the same stack as Bragg, Bragg must be on top--his portrait will be shown on the stack on the map--or if the stack is selected, he is the furthest to the left in the Stack Panel--the highest ranking leader is always on the far left. This is regardless of whether Bragg, or the other leader, or both are commanding a division, as long as Bragg is the highest ranking in his stack.
BTW Armies and Corps are simply stacks in which the highest ranking leader is either an army commander or a corps commander respectively.