Choose the Class Differences... menu item to
  compare the contents of two classes.

  If CMS finds differences, it creates a file that contains
  the generations that differ between them, and delivers a
  copy of the file to your current default directory.

  If the contents of the classes are the same, CMS issues a
  message to that effect and does not create a differences file.
  If you have turned off the Differences Only button, CMS still
  creates a file, even if there are no differences.