VMS Help RTL Routines, LIB$, LIB$FREE_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:       read only
    mechanism:    by reference

    Logical unit number to be deallocated. The logical-unit-number
    argument is the address of a signed longword integer that
    contains this logical unit number, which is the value previously
    returned by LIB$GET_LUN.