VMS Help SET, HOST, Qualifiers, /APPLICATION_PROTOCOL  *Conan The Librarian (sorry for the slow response - running on an old VAX)

       /APPLICATION_PROTOCOL[=protocol]

    Specifies the protocol connection, CTERM (CTDRIVER) or RTERM
    (RTTDRIVER) on the specified node. CTERM protocol is the default
    if no connection is specified.