This puts a SONET/SDH interface into one of several loopback test modes - looping the received signal back out the transmitter (facility loop), looping the about-to-be-transmitted signal back into the local receiver (terminal loop), some other undefined loopback type, or no loopback at all for normal operation. Administrators use loopback modes to isolate whether a fault lies on the local equipment side or further out on the transmission facility, without needing physical test equipment. Setting this object to sonetFacilityLoop would let an admin verify the far-end transmission path is intact by having any signal sent to this interface reflected straight back.