This reports the measured fiber distance to the ONU, typically derived from ranging/RTT measurements the OLT performs as part of normal GPON operation. Admins use it to sanity-check the physical fiber plant, since an unexpectedly large or small distance compared to the known installation can indicate a splicing error, a rerouted fiber, or a misidentified ONU. For example, a distance reading far outside the expected range for a given customer's address would prompt an admin to double-check the fiber routing.