A function that would be tied to how a vehicle is set up in qorkbench, where an assigned "crew" unit is specified (for instance, setting a rifleman as crew for a humvee) and once used, all postions in the vehicle for full function (Driver, Gunner and Commander) are filled by the crew unit (and you can set for instance helicopter crew for helicopters, tank crew for tanks etc.).

This is separate from the "Choose Passengers" function, which is primarily for AI that are not in function positions (like having them be just passengers in a truck, but not a driver).