The UNRESERVE command cancels an existing reservation of a
    generation of an element. You cannot unreserve a generation held
    by another user unless you hold BYPASS privilege or unless you are
    granted BYPASS access to the element by an access control entry.

    If you have more than one reservation of an element, or if
    you are replacing a concurrent reservation made by another user,
    (that is, if there is any ambiguity), you must specify the exact
    reservation to be canceled.  You do this by using either the
    /GENERATION qualifier or the /IDENTIFICATON_NUMBER qualifier.

    You can use /GENERATION as long as the concurrent reservations are
    not on the same generation. If you have more than one concurrent
    reservation for the same generation, you must identify the
    specific reservation to be canceled. Each reservation is assigned
    an identification number. 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. If you use the /IDENTIFICATION_NUMBER qualifier, you
    do not need to also use the /GENERATION qualifier; when both are
    used, CMS ignores the /GENERATION qualifier.