VMS Help SET, VOLUME, Qualifiers, /HIGHWATER_MARKING  *Conan The Librarian (sorry for the slow response - running on an old VAX)

       /HIGHWATER_MARKING
       /NOHIGHWATER_MARKING

    Determines whether the file highwater mark (FHM) volume attribute
    is set. The FHM attribute guarantees that a user cannot read data
    that was not written by the user. Applies to Files-11 On-Disk
    Structure Level 2 (ODS-2) and 5 (ODS-5) volumes only.