/sys$common/syshlp/helplib.hlb MERGE, /CHECK_SEQUENCE, Full Description  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    The /CHECK_SEQUENCE qualifier is unique to the MERGE command. By
    default, Merge does sequence checking to ensure that the input
    files have been sorted on the same key.

    You can also use the /CHECK_SEQUENCE qualifier to check whether
    the records of one or more files (up to 10) have been sorted.
    (The records will still be directed to an output file, which you
    must specify.) If you are checking whether records are sorted on
    a key field other than the entire record, you must specify key
    information, along with requesting sequence.

    High-performance Sort/Merge: The high-performance Sort/Merge
    utility allows you to specify up to 12 files.