Kit U contains 2 wide, freight sized, doors for closing off a wide corridor and 2 ventilation shaft markers. The doorway is approximately 3"x3". The door lifts out in the same way as the small doors, but is in two halves so you can have it partially open for more cover. Use the vent markers in pairs to mark crossing points between parallel corridors or rooms. Vent markers mount on the side walls in the same manner as the up & down ladders.