Bulletin board
Dave's own version of Citadel (DOC) is a variant of the Citadel/UX Bulletin board system (BBS) software which was developed specifically to run ISCA BBS in the late 1980s. It is based on Citadel/UX 3.0 but very heavily modified to suit the specific needs of ISCA BBS, which at its peak was a massive system supporting hundreds of simultaneous users. DOC and its descendants are now used by a number of Internet BBSes, including some in Latin America, which have been heavily modified, support 8-bit characters, and are presented in Spanish.