XDM (the X Window Display Manager) originated in X11R3. This version suffered from several problems, most notably when users switched X terminals off and on. In X11R3, XDM only knew about an X terminal from its entry in the Xservers file, but XDM only consulted this file when it started. Thus every time a user switched a terminal off and on, the system administrator had to send a SIGHUP signal to XDM to instruct it to rescan Xservers
XDM is incompatible with GDM, KDM. It you have XDM runing and type
another X screen is opened (i.e. Screen 1 for xdm, instead of Screen 0, that is used for xdm). You can solve definitively this problem running dpkg-reconfigure xdm.
Configuring XDM; a graphical login interface for Linux or UNIX (Mark Nielsen, [Linux Gazette] #43, July 1999)