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

    If the sort operation is a record or tag sort, the default output
    record format is the same as the first input file record format.
    If the sort operation is an address or index sort, the default
    output record format is fixed record format. If the input files
    have different record formats, Sort provides an output record
    size that is large enough to contain the largest record in the
    input files.

    When you specify the output record format, you can indicate
    the maximum record size, in bytes, of the output records. You
    can specify fixed-length records, variable-length records, or
    variable with fixed-length control records.