VMS Help STOP, /CPU, Parameter  *Conan The Librarian (sorry for the slow response - running on an old VAX)

 cpu-id[,...]

    Specifies a decimal value representing the identity of a
    processor in an OpenVMS multiprocessing system. On a VAX 6000
    system or an Alpha 7000 system, the CPU ID is the backplane slot
    number of the processor. If you do not specify a CPU ID, the
    STOP/CPU command selects a processor in the current active set to
    stop.