From Pilot_51's Wiki
This property defines the visibility and shootability of a soldier in a vehicle. It can be set one or more times within an object. The settings are:
c_SeatIsOutside - means the soldier can be can be shot even though he is in a vehicle. c_SeatShowFullBodySoldier - means that the full body of the soldier can be visible. c_SeatShowHalfBodySoldier - means that at most only the top half of the player should be visible. c_SeatShowHeadOfSoldier - means that just the head of the soldier should be visible. (not used in game) c_SeatShowStandingSoldier - means that a standing soldier will be visible.