Vehicles have up to 3 "active" positions in them:
In addition, some vehicles have Cargo space in which to carry extra units. Tanks and armoured vehicles usually have the three main positions. Aircraft usually have position of Pilot and Gunner.
To put a unit directly into the Commander's seat:
To put a unit directly into the Driver's seat(includes aircraft):
To put a unit directly into the Gunner's seat:
To put a unit into a cargo seat:
Note: Units put into cargo will fill up the cargo space sequentially, but sometimes you may wish to have gaps in the cargo spaces for a more natural look.To achieve this, create a Game Logic and name it, and then move that into cargo just as you would an ordinary unit. The Game Logic will occupy the seat - invisibly - and the next unit to board in cargo will occupy the next sequential seat, creating a more natural gap between units.
With ArmA came the moveInTurret command, to move a unit in to a specific turrent use: