→ Battles can have a maximum of 4 full stack (20 units) armies per side, meaning that large scale coalition battles can have 160 active units at the same time.
→ Player can only control a maximum of 40 units.
→ Just imagine a 4 v 4 multiplayer custom battle in your head, and everything will make sense
→ By setting your personal reinforcement armies to "AI Control", it will treat that army as a separate faction/player.
→ So for example, if you have 3 armies, and you have a 4th allied army, you need to place your 3rd army under "AI control", because you as a faction can only control a maximum of 40 units. By placing your 3rd reinforcement army under "AI Control", the game treats it as another faction or player, like multiplayer. Personally I like to set all my armies to AI control except for just 1.
→ Another tip is to check the turn order of factions by using the "Watch AI moves camera setting" button to best coordinate your movements as well as using the war coordination order, "Defend Army" on either your army or another allied one
→ If you want allied help with sieging, you can also attack a settlement, and stop your army with backspace while it's right next to it, and inside of the Zone of Control. Then fortify or whatever, and next turn your allied army with the "Defend army" War Coordination order should hug the perimeter of the Zone of Control trying to reach you, allowing for a allied siege.

→ When you place your armies under AI Control, the AI will match you. This means that if it's just your 4 armies vs 4 enemy armies from the same enemy faction, they will match the amount of armies you place under "AI Control", or in other words place under "a separate faction/player". Imagine everytime you tick the "AI control" button, they are doing the exact same thing.
→ The implications of this in my opinion favors the player more, because the player relies more on breaking the morale of the AI armies (Battle Difficulty), and by splitting up the AI armies into 4 separate armies or "different factions", each of those "factions" have their own separate Army losses morale mechanic and morale, rather than say 1 large "faction" that constantly receives reinforcements, which means achieving army losses is basically impossible.
→ Another benefit is that by having all 160 active units on the battlefield, it's a lot easier to use "Winds of Magic" spells to absolutely decimate the enemy as there is just so much more stuff on the battlefield. Players usually rely more on magic and are more effective with it than the AI.
→ Lastly, it's just way more fun to have 80 v 80 battles in Warhammer III, it's the thing this mod tries to foster, so you should always try to do it. when possible. Set all of your army reinforcements to AI control and don't control a large army, you should only control 20 units, much simpler and beneficial due to what I said above with army losses, Winds of Magic, and fun.

Player → 20 units
Player's reinforcements #1 (AI CONTROLLED) → 20 units
Player's reinforcements #2 (AI CONTROLLED) → 20 units
Ally #1 → 20 units

vs.

Enemy #1 → 20 units
Enemy #2 → 20 units
Enemy #3 → 20 units
Enemy #4 → 20 units

Player → 20 units
Ally #1 → 20 units
Ally #2 → 20 units
Ally #3 → 20 units

vs

Enemy #1 → 20 units
Enemy #2 → 20 units
Enemy #3 → 20 units
Enemy #4 → 20 units

Player → 20 units
Player's reinforcements #1 (Control Large army enabled, no AI Control) → 20 units
Player's reinforcements #2 (AI CONTROLLED) → 20 units
Ally #1 → 20 units

vs

Enemy #1 → 20 units
Enemy #2 → 20 units
Enemy #3 → 20 units
Enemy #4 → 20 units


Player → 20 units
Player's reinforcements #1 (Control Large army enabled, no AI Control) → 20 units
Player's reinforcements #2 (Control Large army enabled, no AI Control) → 20 units
Ally #1 → 20 units

vs

Enemy #1 → 20 units
Enemy #2 → 20 units
Enemy #3 → 20 units
Enemy #4→ 20 units

https://steamcommunity.com/sharedfiles/filedetails/?id=3330052990
https://steamcommunity.com/sharedfiles/filedetails/?id=3330053200
https://steamcommunity.com/sharedfiles/filedetails/?id=3330053572
https://steamcommunity.com/sharedfiles/filedetails/?id=3330053647
https://steamcommunity.com/sharedfiles/filedetails/?id=3330053726