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

    Creates a user-mode display device and assigns the logical name
    DECW$DISPLAY to point to it. This qualifier must be used with
    the /CREATE qualifier. The lifetime of a user-mode device is
    one DECwindows image: when the next DECwindows image exits, the
    device is deleted.