This read-write enumeration indicates which FR-USER port serves as a reference port for conditional LMI responses on the FR-NET port; if set to none, the FR-NET port always answers LMI requests, but if a specific port is specified, FR-NET only answers LMI requests when that reference port itself is getting answers to its own LMI requests. An administrator uses this to make the network-side LMI response conditional on a specific user port's own link health. For example, configuring this to reference a critical user port would stop the FR-NET side from claiming LMI health if that critical port itself isn't getting valid LMI responses.