The Delete Generation... menu item removes information
  about one or more generations of an element. You may want
  to delete some generations of an element if your library
  contains a large number of unneeded generations of an
  element.

  Once a generation is deleted, it cannot be restored to
  its former place in the element in the CMS library.
  You can, however, permanently store the contents of
  the generation using the Archive file button.

  When you choose Delete Generation..., a dialog box
  appears, allowing you to specify options for the
  generation you are deleting.