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 bin is the still-open, currently-accumulating interval, its value is only a partial count until the bin's Duration elapses and it rolls into history -- polling it twice a few minutes apart is the fastest way to catch a fault building up in near real time. A count of 3 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.