After you have passed either the file names or the records to
    SORT, call the SOR$SORT_MERGE routine to sort the records. For
    file interface on input, the input files are opened and the
    records are released to the sort. For the record interface on
    input, the record must have already been released (by calls
    to SOR$RELEASE_REC). For file interface on output, the output
    records are reformatted and directed to the output file. For the
    record interface on output, SOR$RETURN_REC must be called to get
    the sorted records.

    Some of the return values are used with different severities
    depending on whether SORT/MERGE can recover. Thus, if you want to
    check for a specific status, you should use LIB$MATCH_COND.