VMS Help RTL Routines, SMG$, SMG$INVALIDATE_DISPLAY, Argument  *Conan The Librarian (sorry for the slow response - running on an old VAX)

 display-id

    OpenVMS usage:identifier
    type:         longword (unsigned)
    access:       read only
    mechanism:    by reference

    Specifies the virtual display affected. The display-id argument
    is the address of an unsigned longword that contains the display
    identifier.

    The display identifier is returned by SMG$CREATE_VIRTUAL_DISPLAY.