VMS Help RUNOFF, /INDEX, Parameter  *Conan The Librarian (sorry for the slow response - running on an old VAX)

 filespec[,...] or filespec[+...]

    Specifies one or more intermediate binary files (.BRN) that
    contain information (index entries, page number references, and
    so on) for making an index. To create a .BRN file, use the RUNOFF
    command with the /INTERMEDIATE qualifier. See the RUNOFF command
    for more information on the /INTERMEDIATE qualifier.

    If you omit the input file type, the DSR Indexing utility uses
    a default file type of .BRN. The RUNOFF/INDEX command also
    processes .BIX files that the previous version of DSR produced.

    For single input files, the DSR Indexing utility produces an
    output file with the same file name as the input file. The output
    file type is .RNX.

    If you separate multiple input files with commas (,),  separate
    .RNX files for each input file are created. If you separate
    multiple input files with plus signs (+),  a single .RNX file
    that contains indexing information for all of the input files
    is created. The default output file name is the same as the
    first input file name; the default file type is .RNX. Wildcard
    characters are not allowed in the file specification.