VMS Help CMS, UNRESERVE, Qualifiers, /GENERATION=generation-expression  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    Specifies which reserved generation of the element is to be
    unreserved. If you have more than one reservation of the same
    element generation, you must use the /IDENTIFICATION_NUMBER
    qualifier to unreserve the reservation.