This field records the port number reported by the device at the far end of this FC monitoring session's destination port's physical link. For the physical FC port designated to receive the mirrored/copied traffic for an FC monitoring session, comparing local and peer port numbers lets UCS Manager render an accurate topology diagram, and a peer port number that keeps changing between polls can itself be a symptom of a flapping or misbehaving link partner. A stable link here would report the same peer port number, say port 36, on every poll rather than jumping around.