Sometimes called external programs, doors are the most common way to add games, utilities, and other extensions to BBSes.
Most doors were responsible for operating the serial port or other communications device directly until returning control to the BBS.
[7] There were also role-playing games (RPG), often derived from earlier email-based games—examples include Seth Robinson's Legend of the Red Dragon,[8] popular dystopian RPG: Operation: Overkill,[4] and Mutants!.
While many of the most popular and memorable BBS doors have been games, numerous doors had non-entertainment applications such as user polls or the time bank, permitting users to time-shift their rationed BBS use.
Frequently they act as a front-end to themed databases on subject such as astrology, numerology and fortune-telling, recipes, weather prediction, personal ads (sometimes with additional match-making functionality), classified ads and "for sale" listings (sometimes permitting auctions), BBS lists, and parting comments from the most recent BBS callers.