This counts Section-layer Severely Errored Seconds on this SONET/SDH port: seconds where the Section-layer error rate crossed the severe threshold during the bin, per the MIB's description. SESS on this port reflects the rawest, closest-to-the-fiber severe-error measurement available for this section -- because it's measured below the Line layer, it's often the first counter to move when a physical-layer fault (fiber, connector, transceiver) develops on this specific hop; because this specific bin belongs to the completed-interval history table, its value is frozen the moment the interval closes, so re-polling it later will return the same number -- useful for pinpointing exactly which past 15 minutes a problem occurred in. A count of 1 severely errored seconds at the Section layer in this bin would prompt an admin to check this port's actual received optical power before looking any further up the protocol stack.