VMS Help RTL Routines, SMG$, SMG$DELETE_VIEWPORT, 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

    Identifier of the virtual display associated with the viewport to
    be deleted. The display-id argument is the address of an unsigned
    longword containing the display identifier.