Fill in the Element field with the name of the object
  (or objects) you want to reserve.  By default, CMS
  reserves the most recent generation on the main line
  of descent of each element.

  The object can be an element name, a group name, a
  wildcard expression, or a list of these separated by
  commas.

  If you want to reserve a specific generation of the
  element, you must specify the generation number in the
  Generation field.