Two units from different sides (eg. blufor vs. opfor) cannot enter the same vehicle. If a blufor unit enters a vehicle, an opfor cannot enter it. Or if an opfor enters a vehicle, a blufor unit cannot enter it.

This causes 2 problems. One being that, well, you just can't get in with your enemies (Eg. prisoner transport).

Another problem is, that if you accidentally kill a few civilians for example, or a few allies, you will become an enemy to that side, and thus, cannot enter your friend's vehicle, even though you are (in spirit) on the same side.