I thought this was required by the FAA advisory circular governing parachute operations (https://www.faa.gov/documentLibrary/media/Advisory_Circular/AC_105-2E.p

I thought this was required by the FAA advisory circular governing parachute operations (https://www.faa.gov/documentLibrary/media/Advisory_Circular/AC_105-2E.pdf https://www.faa.gov/documentLibrary/media/Advisory_Circular/... - PDF), but after just looking, I now think it was part of the supplemental type certificate (https://www.faa.gov/aircraft/air_cert/design_approvals/stc/ https://www.faa.gov/aircraft/air_cert/design_approvals/stc/) or 337 approval for the door modifications on the planes at the drop zone where I used to work. Thus, it probably depends on the aircraft, and possibly the process by which the owner got FAA approval to modify the door.