This PerfCounter64 counts Severely Errored Seconds encountered by an E3/T3 interface's line layer in the history bin. An admin uses this line SES history, on a trunk that often aggregates many downstream circuits, as a high-priority indicator for scheduling maintenance windows. If e3t3HistoryLineSESs shows repeated spikes at the same time each day, the admin would suspect a time-correlated environmental cause, such as temperature swings affecting an outdoor cable run, rather than a random fault.