This counts generic Errored Seconds on this SONET/SDH port at whatever aggregate signal level this bin applies to: one-second intervals with at least one error event, per the MIB's description. This generic ES rolls up the port's overall error-second tally for this bin (as distinct from the Line- and Section-specific ESL/ESS counters found elsewhere in this same table), giving an admin a single top-level number to watch for this port before drilling into which specific layer is responsible; 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 value of 17 here for this bin, compared against ESL and ESS for the same interval, would tell an admin at a glance which layer to investigate first on this port.