Pan Am Boeing 747 loading cargo into forward cargo bay.
Pan Am 103 reconstruction showing area of destruction which matches UAL 811 below.
The door seal is a fabric impregnated with a rubber/silicone
substance that is shaped like an angled T. The top of the T is
press fitted into an aluminum c channel riveted to the door frame.
The free flange of the T is compressed against an aluminum V type
channel called a seal depressor on the fuselage.
Air pressure trying to blow this seal out pushes it harder against the V channel. Cargo containers often damage the seal depressors, especially in the top corners and the seals let a lot of air pass. It is not a safety concern other than it reduces fuel efficiency by requiring more bleed air from the engines to maintain a cabin altitude.
Boeing 747 nose picts right side cargo door
AD Airworthiness Directive 79-17-02
Airworthiness Directive Amendment to 89-05-54 amending 88-12-04
Airworthiness Directive 88-12-04 Boeing 747
More pictures of UAL 811 cargo door hole
Cargo door accidents