csOc3LinePrevDaySESs 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 during all of yesterday, frozen for day-over-day comparison. 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, during a morning health check an admin compares yesterday's frozen csOc3LinePrevDaySESs value against the day before to see whether 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 is trending better or worse.