/sys$common/syshlp/helplib.hlb SET, 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 is the backplane slot
    number of the processor.