VMS Help MERGE, Specification File Qualifiers, /STABLE, Full Description  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    By default, when records are sorted with identical keys, the
    order of those records in the output file may not be the same as
    they appeared in the input file. Specifying the /STABLE qualifier
    in a specification file arranges records with equal keys in the
    output file in the order of the input files as specified in the
    command line. If you use this qualifier when sorting multiple
    input files, on output, records with equal keys in the first file
    will precede those from the second file and so on.