/sys$common/syshlp/helplib.hlb MOUNT, Qualifiers, /HDR3  *Conan The Librarian (sorry for the slow response - running on an old VAX)

       /HDR3 (default)
       /NOHDR3

    Controls whether ANSI standard header label 3 is written on a
    magnetic tape volume.

    By default, header label 3 is written. You can specify the
    /NOHDR3 qualifier to write magnetic tapes that are to be used
    on other systems that do not process HDR3 labels correctly.

    Example

    In the following example, the INITIALIZE and MOUNT commands
    prepare an ANSI-formatted magnetic tape for processing. The
    /NOHDR3 qualifier specifies that no HDR3 labels are to be
    written, thus creating a magnetic tape that can be transported
    to systems that do not process implementation-dependent labels
    correctly.

    $ INITIALIZE  MTA0: ABCD
    $ MOUNT/NOHDR3 MTA0: ABCD