Bakugan Mobile Assault Vehicles (BMA) are some kind of Bakugan Battle Gear, although there are some major differences. Battle Gear are typically attached on top of the Bakugan, but with Bakugan Mobile Assault, it is the Bakugan that is placed on top (they can be seen as a vehicle). Also, unlike with Bakugan Battle Gear, it seems that the colors (attributes) of the BMAs follow the six Bakugan attributes (Pyrus, Aquos, Haos, Ventus, Subterra and Darkus). The attributes of Battle Gear are Gold, Silver and Copper. In Bakugan Dimensions, Mobile Assault Vehicles are used as normal Battle Gear.
You can use a maximum of two BMAs in your battle force (or a maximum of two of any combination of BMAs, Battle Gear or Traps). They each come with a Reference Card that lists their Special Abilities, and also what type of Gate Card (Gold, Silver and/or Copper) they can be used on.
After you put your Bakugan on your Bakugan Mobile Assault, you roll it onto a Gate Card. The Bakugan gains the BMA's G-Power only on the types of Gate Cards listed on the BMA's Reference Card. If the Bakugan Mobile Assault is used on a Gate card NOT specified by the Reference Card, the BMA is destroyed and must be removed from the game entirly (the Bakugan is used).
We have seen three Mobile Assault surface so far:
The Impalaton, Jakalier, and Raytheus packs contain a Bakugan Mobile Assault Vehicle (Deluxe Battle Gear), 1 Ability Card, and 1 Gate Card. They were officially released by Spinmaster in October 2010. (credits to Bakugan.wikia.com for the images)