VMS Help DIFFERENCES, Qualifiers, /WINDOW  *Conan The Librarian (sorry for the slow response - running on an old VAX)

       /WINDOW=size

    Searches the number of records specified by the size parameter,
    before a record is declared as unmatched. By default, the
    DIFFERENCES command searches to the ends of both input files
    before listing a record as unmatched.

    The window size is the minimum size of a differences section
    that will cause the DIFFERENCES command to lose synchronization
    between the two input files.