Currently by the rules posted by Steve:
* Light AA will only fire if it is directly attacked, in other words is part of the formation attacked by enemy fighters.
* Medium AA will only fire if a sub-ordinate formation, in a direct line, is attacked by enemy fighters or in above situation.
* Heavy AA will fire in both above situations, but also will take potshots freely at any enemy fighters (ground support, bombardment, CAP).
This means that creating special AA-only formations only works with Heavy AA. If we want to shield our ground units from enemy fighters, Light AA needs to be in each formation and Medium AA will need to be directly attached to a superior formation.
So, creating an AA battalion for a division is counter-productive if it consists of Light or Medium AA. Instead, players should create AA units out of Heavy AA, put Medium AA together with division/battalion HQs (or whatever equivalent they are using) and put Light AA into front line units where it can both improve ground combat as well as help bleed out enemy fighters.