This reports which loopback state, none, line, or diagnostics, is actually in effect right now on this SONET/SDH port. As the read-only counterpart to pportSonetSDHLoopbackConfig, checking this confirms whether a requested test loop actually engaged on this port, or reveals that a loop was left active and is why the circuit is unexpectedly not passing traffic. Finding this port stuck at lineLoop when no test should be running would explain a full outage on that SONET circuit that is not visible as a simple link-down condition.