I understand what it all does,its just that from what i've read the unit in the bustle rack at the back,like on the model I have is the air conditioning unit or TMS that control temps inside the tank and it incorporates the VCSU and the unit on the rear is the APU.If thats the case then the model I have has'nt got 2 APU's.I also noticed that VS Tank sell the accessories to upgrade to the A2 and they advertise that unit that goes in the bustle rack as an air conditioning unit.From all this different info i'm getting confused to what the unit at the back of the turret on my tank really is and according to the literature i've been reading on the Abrams its a TMS air con unit and not an APU,it says the APU is mounted on the left rear sponson fuel cell area.My brain is starting to hurt...lol...This is where I got the info from...[link=http://www.globalsecurity.org/military/systems/ground/m1a2.htm]Abrams info[/link]