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

    TAPE_ALLOCLASS determines the tape allocation class for the
    system. The tape allocation class creates a unique clusterwide
    device name for multiple access paths to the same tape.

    The TAPE_ALLOCLASS parameter can also be used to generate a
    unique clusterwide name for tape devices with identical unit
    numbers.