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 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 value of 8 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.