This is the identifier, per CucsNetworkSwitchId, of the specific peer switch discovered on the other end of this fabric interconnect's cabling. For this row, it tells you concretely which switch, e.g. fabric interconnect A versus B if they're cross-connected, or an external device, UCS Manager has actually detected on the wire, which is what you'd check to confirm the physical topology matches the intended design. An id value indicating fabric interconnect B here, on a cabling record you expected to show an external upstream switch, would reveal an unexpected direct A-to-B cross-connection instead.