Older X terminals may not support XDMCP. For the login server to display a login screen on this type of X terminal, list the X terminal name in the Xservers file.
Since the display is on the network, display_name includes the host name as part of the name. The display class can be used to specify resources specific to a particular class of X terminals. (Your X terminal documentation should tell you the display class of your X terminal.) The display_type of foreign tells the login server to connect to an existing X server rather than to start its own. In this case, an X_server_command is not specified.
The following lines in the Xservers file direct the login server to display a login screen on two non-XDMCP X terminals, ruby and wolfie:
ruby.blackdog.com:0 AcmeXsta foreign wolfie:0 PandaCo foreign