I believe that in the AB all units have to be assigned to a sub-commander - so no independent units on setup. That does not mean the units have to be in the sub-commanders command range for deployment though, just that you can't have AB troops which start independent to avoid poor vigor impacts for their potential sub-commanders.
During the game, if units are out of range of the sub-commander but are still within the range of the commander then they are not out of command. There are no rules which force you to have units stay within a sub-commanders command range but there is a penalty for not being in command.
This allows the defender to have 1 more functional groups of troops than the number of sub commanders they have. That is those under the sub-commander(s) plus one group for those who have direct command from the overall commander. This also allows you, to in your terms, 'drop off' reserve troops which may be controlled by the overall commander, as he is in their chain of command- their boss's boss so to speak. However, those troops are still impacted by the vigor roll of their sub-commander and tactics impacts if they are in range of their sub-commander.
Interestingly, for some armies such as the Ottomans with poor vigor rolls and all amateur troops, putting the sub-commanders on the fringes of the battle, and letting the commander take an active part can significantly help the army. I should do this next time... :)
Well, that's how I understand the rules and I'm sure others will comment if I'm wrong.I hope it helps.