This read-only enumerated value reports which kind of loopback is configured on this OTU4 (about 111.8 Gbit/s) line-side OTN facility: none, terminal, or facility, defaulting to none. A terminal loopback returns traffic back toward the client side while a facility loopback returns it back toward the line side, so the choice determines exactly which segment of the path is being tested. For example, an operator suspecting a fault on the line side of the OTU4 facility on slot 3 would use a facility loopback there rather than a terminal loopback, which would only test the client-facing segment.