In writing these posts up its occurred to me that the way in which units operate in the AGE system could do with some explanation. So here goes:
1) What is an army
Well it is one of two things. It is a collection of an army commander (who maybe stacked on their own or with other units) and a number of corps that he controls.
Here's an example:

This is Napoleon's Army from the Waterloo scenario (its the same game as I used to show the rules on frontage. So I've re-organised to suit me. Less corps than the OOB starts with and a strong reserve in with Napoleon. As under stack commitment, the CinC's own stack will commit in support of an already engaged stack (in other words I have a good chance of winning that part of the overall battle decisively).
But an army can also be a HQ without linked units. Here is Archduke Karl at the start of the 1805 campaign.

He has no control over the other stacks (even though they are notionally part of the army of Italy). Your only real choice is to put as many into one huge stack with him directly leading it.
2) What is a stack?
To me, a 'stack' is any of the individual brown tabs you see. So it can be army, a division, a single leader. After re-organising Karl then a stack would also be the army.
This matters as some commanders have traits that apply to the stack they are physically in (and others only if they are the senior officer in that stack).
So a stack is any unit that you would directly move on the map.
3) Divisions etc
Wars of Napoleon uses two different things that can be treated as a division. In some scenarios, you have a commander and a fixed set of units which form a division. The Waterloo scenario has examples of this, for example the second division. You can see how it is made up but you can't break it down.

more typically is a 'division' made up of one of more buildable units that you construct and can re-organise as you want:

so here the Rechte Division can be broken down and re-assigned.
So the second tier of organisation is anything you have assigned to a stack. Can be a unit that can't be broken down or something you have constructed.
4) Battalions etc – in other words the 'element'
Finally we come to a level that never actually appears on the map. You can see these by clicking on the various 'elements' within a larger unit.

However, the element is not the smallest unit in the game. Each is composed of a number of companies or similar. In the case of a French line battalion there are 8 of these. They in turn form the 'hit points' used to apportion losses.
As you can see the battalion above has been battered in the last battle and has lost half its strength. Its those missing hit points that will be replaced using replacement chits. As long as some part of the battalion still exists this is relatively straight forward. If the entire battalion was wiped out, it can be replaced but you need to put the unit it was part of on a depot (preferably in passive posture) and it may take a few turns for the unit to recover.