Basically the opposing pair to https://community.bistudio.com/wiki/assignedVehicleRole and https://community.bistudio.com/wiki/emptyPositions.
A vehicle position can be assigned only to one unit. Currently its fairly tricky and not fully reliable to determine what positions are already assigned and to whom (one can loop through all vehicles with assignedVehicle and assignedVehicleRole).
So far its a big point of frustration in managing AI units to use vehicles as you often run into situations where AI wont enter a vehicle position empty, whereas its already assigned to a different unit.
This getting would make vehicle use by AI via scripting (especially in combination with waypoints to make AI enter/leave vehicles) a lot easier and manageable.