Choose the Element/File Differences... menu item to
  compare two files, two generations of elements, or
  a file and a generation of an element.

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

  If the files 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.