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

    Use this parameter for Phase II shadowing only. The SHADOW_SYS_
    UNIT parameter is an integer value that contains the virtual unit
    number of the system disk. The default value is 0. The maximum
    value allowed is 9999. This parameter is effective only when the
    SHADOW_SYS_DISK parameter has a value of 1. This parameter should
    be set to the same value on all nodes booting off a particular
    system disk shadow set. See Volume Shadowing for OpenVMS for more
    information about setting system parameters for volume shadowing.