None, anymore.
During the heyday of the dialup BBS, there were many different implementations of software called "Citadel" which provided a similar feature set and command set. Our variant was called "Citadel-UX" because it runs on Unix compatible operating systems.
Our implementation (along with its forks) is the only one which evolved into a multiuser, Internet-facing system and has been modernized into a groupware and content management system. As of this writing it is the only Citadel implementation still being actively maintained. Therefore, we now refer to it simply as Citadel.