This is the outer door of the USS Sephora
The inner door must be closed and the Airlock depressurized before the outer door can be opened
Red light means the door cannot be opened yet (since chamber has not yet been depressurized) Once the inner door shuts and the compartment is sealed, depressurization occurs and the green light lights, after which the outer door opens.
The Outer Door opens to a Docking Tunnel that was erected between the USS Sephora and the USS Sulaco so that Marines from the Sephora can board the Sulaco
