This counts Section-layer Errored Seconds on this SONET/SDH port: one-second intervals with at least one Section-layer (regenerator section) defect during the bin, per the MIB's description. Section-layer ES is measured one layer below the Line, closer to the raw optical signal on this port, so comparing ESS against ESL for the same bin tells an admin whether a problem is confined to the physical Section (e.g. this specific regenerator hop) or is also affecting the multiplexed Line signal; 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. An ESS reading of 6 for this bin that doesn't show up in the port's Line-layer ES for the same interval would point at a Section-only issue local to this regenerator hop.