The Secondary Input Element/File field specifies the
  second file to be compared.  CMS assumes that the file
  is an OpenVMS file specification.  To specify an element, you
  must specify the exact generation number of the element
  by choosing the Generation button and filling in the
  Generation field.

  CMS follows these rules when you do not provide a second
  file specification:

  o  If you direct CMS to take the Primary Input
     Element/File from a location that is not a CMS
     library, CMS uses the next lower file version in
     the same directory as the Primary Input Element/File.

  o  If you direct CMS to take the Primary Input
     Element/File from a CMS library (by specifying the
     generation number) CMS uses the latest default
     directory version of the Primary Input Element/File
     as the Secondary Input File.