VMS Help Sys Parameters, DR_UNIT_BASE  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    (Alpha only) DR_UNIT_BASE specifies the base value from which
    unit numbers for DR devices (DIGITAL StorageWorks RAID Array 200
    Family logical RAID drives) are counted.

    DR_UNIT_BASE provides a way for unique RAID device numbers to
    be generated. DR devices are numbered starting with the value of
    DR_UNIT_BASE and then counting from there. For example, setting
    DR_UNIT_BASE to 10 produces device names such as $1$DRA10,
    $1$DRA11, and so on.

    Setting DR_UNIT_BASE to appropriate, nonoverlapping values on all
    cluster members that share the same (nonzero) allocation class
    ensures that no two RAID devices are given the same name.

    DR_UNIT_BASE is a GEN parameter.