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

 logical-unit-number

    OpenVMS usage:longword_signed
    type:         longword integer (signed)
    access:       write only
    mechanism:    by reference

    Allocated logical unit number or -1 if none was available. The
    logical-unit-number argument is the address of a longword into
    which LIB$GET_LUN returns the value of the allocated logical
    unit. LIB$GET_LUN can allocate logical unit numbers 100 through
    119 on VAX, and 100 through 299 on Alpha.