I tried playing, maybe someone can explain: in level 2 of Leadership, there's a skill that allows you to use 9 stacks in battle, but it doesn't seem to be working.
I thought this skill was a must-have, but it seems like it doesn't work in regular battles.
I suspect it might only work during sieges, but I haven't reached that point yet.
Can anyone clarify?

It works, allowing you to detach two additional units. However, it often glitches: the slots for the extra stacks can be blocked (this can be fixed by pressing the auto-arrangement button and then manually moving them where you need them) or the detached units can simply disappear into thin air (I've only figured out how to avoid this: you need to have 3 empty slots at the beginning of the arrangement and start with the middle one, then the far right one, and finally the remaining one).