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 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. An ESS reading of 14 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.