This PerfCounter64 counts Severely Errored Seconds encountered by an E3/T3 interface's line layer in the current bin. Because a DS3/E3 trunk often aggregates many lower-speed circuits, an admin treats a live rise in line SES here as a higher-priority event than the same counter on a single low-speed circuit. If e3t3StatsLineSESs starts climbing on a DS3 trunk carrying multiple T1 circuits multiplexed inside it, the admin would escalate immediately given the number of downstream customers potentially affected.