VMS Help Sys Parameters, TTY_DEFPORT  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    TTY_DEFPORT provides flag bits for port drivers. Bit 0 set to 1
    indicates that the terminal controller does not provide automatic
    XON/XOFF flow control. This bit should not be set for Compaq
    controllers, but it is needed for some foreign controllers.
    Currently only the YCDRIVER (DMF32, DMZ32) uses this bit.
    The remaining bits are reserved for future use. This special
    parameter should be modified only if recommended by Compaq.