This identifies which of the redundant fabric interconnects (typically 'A' or 'B') currently manages or carries this endpoint. For this blade's physical port into the chassis backplane fabric, switchId is the field that tells you which side of a dual-fabric-interconnect deployment owns this specific link, which is exactly what you check when only half of a chassis's redundant paths goes down. A value of 'B' here means this endpoint's traffic currently rides through fabric interconnect B, not its A counterpart.