This field records the port number reported by the device at the far end of this SAN uplink port's physical link. For one physical FC or FCoE uplink port that is a member of the fabric interconnect's SAN cloud, 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 33, on every poll rather than jumping around.