This integer identifies which CPU a given memory module is associated with, where 0 means the memory is not tied to any particular processor. It matters on NUMA-style servers where memory performance and errors are often local to a specific CPU socket, so knowing the CPU number helps pinpoint the source of a memory problem. For instance, if modules under CPU number 1 keep reporting issues while CPU 0's modules are clean, that points suspicion at CPU socket 1's memory riser.