With the introduction of Main Battle Tanks in Alpha 12 came the Crewman class. Equipping the Crewman kit grants access to many of the heavier vehicles in Squad. Infantry Fighting Vehicles and Main Battle Tanks require every crew position to be manned by a Crewman. Armored Personnel Carriers generally require a Crewman to drive the vehicle but only require a Crewman if the gunner is in an enclosed turret. Please see the Vehicles page for a complete overview of which vehicles require a Crewman to crew (number next to steering wheel).
As of Alpha 13, players can change to a crewman kit from a vehicle. This means that if a vehicle has the necessary ammo points, a player can switch to a crewman kit in order to crew a vehicle. Note that light vehicles and Main Battle Tanks only allow the Crewman kit to be selected.