<AllowGrouping>0</AllowGrouping> sets if it is trained alone or in groups
<IsChampion>1</IsChampion> for whether it can equip and trade gear
<GetPerksOnLevelUp>1</GetPerksOnLevelUp> for whether it can get traits on levelup (possibly overriden for champions to always be 1)
Interesting... So you could have a "single" version of trainable units by duplicating the XML for that unit and adding the <AllowGrouping>0</AllowGrouping>, which could open a lot of possibilities for the early game and also for very expensive units later.
And GetPerksOnLevelUp... Works for trainable grouped units? Opens up a lot of possibilities too!