Fill in the Generation field with the number of the
  specific generation to be replaced; for example, 3B2.

  Choose the Generation option when you have more than one
  reservation of an element or if you are replacing
  another user's reservation (you can also do this by using
  the Reservation Identification option).

  You can use Replace 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 fill in the Replace Reservation
  Identification option.