VMS Help CMS, REPLACE, Qualifiers, /IDENTIFICATION_NUMBER=n  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    Specifies which reservation is to be replaced. This qualifier
    is required when you have multiple reservations of the same
    generation of an element. /IDENTIFICATION_NUMBER can be used
    instead of /GENERATION when you have multiple reservations. Use
    the SHOW RESERVATIONS command to determine the identification
    number of each reservation. The identification number appears in
    parentheses at the beginning of each line.