"Control large army" option

[FelipepRanger]

I'm not sure what it does, it says you can control up to 40 units without needing to wait for reinforcements but when I enter the battle using it my reinforcements are still off-screen

[Elitewrecker PT]

Without it ticked you can only control 20 units. But you’ll always have to wait for the reinforcements to arrive.

[grognardgary]

Is it possible that it only works if you sally as part of a siege?

[WarMage]

Your reinforcements will always be off screen. Only the two main armies are present during deployment.

With the option off
You will never control more than 20 units during the battle. The extra units will be considered as a reserve and will replenish your losses during the battle.

With the option on
Your other armies will, eventually, join the battle. You'll be able to have up to 40 units on the map. Any extra will join the reserve and act as previously explained. Meaning that you can have 2 full armies on the map at the same time.

As far as I see it, that option should always be enabled. It's the best way to play. Sadly enough, you can't select which units will join so if you only have space for 8 more units you may receive 8 basic front line holders instead of your elite cavalry or artillery that would've been much more useful at the time. It's a bit sad. You can get screwed hard by RNG.

[Elitewrecker PT]

It's not RNG. The reinforcement troops enter the battle in the same order that they're displayed in the army in the campaign map - i.e. lords and heores first, then infantry, cav and artillery last.
And the reinforcement units are split evenly between multiple reinforcing armies.

So yeah, if you want to run around with 3 armies together, you need to stack the artillery on one of them to make sure they enter. Otherwise you'll just get a ton of infantry at the start.