This is the relative name of this physical FC port, the short local identifier distinguishing it from other ports under the same slot or fabric interconnect. Admins and scripts use this to pick out one specific port once they've already navigated to its parent slot, without needing to re-parse the entire DN each time. For example, an RN of 'switch-fc-2' identifies the second FC port in that slot, as opposed to switch-fc-1 or switch-fc-3.