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

    If the organization of the output file is to be different from
    that of the input files, you must specify the new organization.
    If you do not specify file organization, the default for record
    and tag sorts is the organization of the first input file. If you
    do not specify file organization, the default organization for
    address and index sorts is sequential.

    Use the /SEQUENTIAL qualifier when the default is not sequential
    file organization and you want an output file with sequential
    file format.