Compares the contents of two classes. If the particular generation
    of any element, that is a member of the class, differs between the
    classes, CMS creates a file that contains a listing of the class
    members that differ between them. If the contents of the classes
    are the same, CMS issues a message to that effect and does not create
    a differences file (unless the /FULL qualifier is in effect). The
    /CLASS qualifier must be specified prior to any other qualifiers.

       Format:

          DIFFERENCES/CLASS  class1 class2