This PerfCounter64 counts Severely Errored Seconds on a SONET/SDH Line for the history bin, i.e. seconds where the line's bit-error density crossed the severe threshold rather than just a single detected anomaly. SES on the Line layer is what carriers use in SLA math, so an admin tracks it to know whether a circuit is at risk of breaching its errored-seconds performance objective for the period. If ocnStmHistoryLineSESs shows 12 severely errored seconds in a single history bin while ES for the same bin is only slightly higher, that ratio points to a burst fault (e.g. a brief fiber cut) rather than steady low-level noise.