VMS Help DECnet-Plus, DCL Commands, SET, HOST, /X29, Qualifiers, /USER_DATA=string  *Conan The Librarian (sorry for the slow response - running on an old VAX)

       /NOUSER_DATA

    (Default=/NOUSER_DATA)

    Use this qualifier to specify a string of user data that is  sent
    to the remote DTE when the call is established.  The maximum size
    of the string is 12 characters,  unless   /FAST_SELECT  has  been
    specified, in which case it is 124 characters.