VMS Help MERGE, Parameters  *Conan The Librarian (sorry for the slow response - running on an old VAX)

 input-file-spec [,...]

    Specifies the file or files to be sorted or merged. You can
    specify up to 10 input files. Multiple file specifications must
    be separated by commas. The default input file type is .DAT.

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

 output-file

    Specifies the file to be created. You can specify one output
    file only. If you omit a file type in the file specification, the
    command defaults to the file type of the first input file.