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

       /STRUCTURE=level

    Specifies whether the volume should be formatted in Files-11
    On-Disk Structure Level 1, 2 (the default), or 5.

    Structure Level 1 is incompatible with the /DATA_CHECK and
    /CLUSTER_SIZE qualifiers. The default protection for a Structure
    Level 1 disk is full access to system, owner, and group, and read
    (R) access to all other users.

    Note that Alpha does not support Files-11 On-Disk Structure Level
    1 disks, and specifying 1 on Alpha results in an error. VAX does
    not support Structure Level 5 disks, and specifying 5 on VAX
    results in an error.

    Refer to the OpenVMS Guide to Extended File Specifications for
    more information about Structure Level 5 (ODS-5) disks.