csOc3LineDaySESs is a read-only PerfCurrentCount counter reporting the number of Severely Errored Seconds — a one-second period with a very high error density — that the SONET/SDH Line layer of this OC3 port has recorded accumulated so far today. A rising count here is the operational signal that a serious optical-signal fault on the line, such as a dirty fiber connector or a degrading laser is happening on that interface, which is exactly the kind of degradation a monitoring admin wants an early warning of before it causes an outage. For example, checking csOc3LineDaySESs mid-afternoon and finding it already well above a typical day's baseline is an early sign of a serious optical-signal fault on the line, such as a dirty fiber connector or a degrading laser on the SONET/SDH Line layer of this OC3 port, well before the day's tally is complete.