Choose the Reservation Identification option when you
  have multiple reservations of the same element
  reservation (you can also do this by choosing the Replace
  Generation option).

  If you have more than one concurrent reservation for the
  same generation, you must identify the specific
  reservation to be replaced. If multiple users have
  reserved the same generation of an element and you have
  BYPASS privilege or have been granted BYPASS access to
  the element by an access control list entry, you must
  identify the specific reservation to be replaced.
  Each reservation is assigned an identification number.

  Fill in the Reservation Identification field with the
  specific number of the reservation to be replaced; for
  example, 2.

  To determine the identification number of each
  reservation, do the following:

  1. Pull down the View menu.
  2. Choose the Reservation menu item.
  3. Double click on the reserved element to
     expand it into individual reserved generations.

  The identification number appears in parentheses at the
  beginning of each line that lists a reserved generation.