The 3rd occupancy seat or jumper seat mounts on the back circuit wall of the flight deck. The 3rd occupant, an observer, usually sits in this seat or if installed in the aircraft, on the 4th occupancy seat.
It contains a sliding mount to allow the seat to slide to the centre of the cockpit when required. When the seat is down, the back cushion props up to provide lumbar support. The seat can be folded up when in the stowed position.