/sys$common/syshlp/helplib.hlb CMS, DIFFERENCES, file, Command Parameters  *Conan The Librarian (sorry for the slow response - running on an old VAX)

 file1

    Specifies the first file that is to be compared. If you specify
    /GENERATION for this parameter, file1 must be an element name;
    otherwise, a VMS file specification is assumed.

 file2

    Specifies the second file that is to be compared. If you specify
    /GENERATION for this parameter, file2 must be an element name;
    otherwise, a VMS file specification is assumed.

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

    o  If you direct CMS to take file1 from a location that is not a
       CMS library, CMS uses the next lower file version in the same
       directory as file1.

    o  If you direct CMS to take file1 from a CMS library (by
       specifying /GENERATION), CMS uses the latest default directory
       version of file1 as the second input file.