csOc3SectionPrevDayESs is a read-only PerfCurrentCount counter reporting the number of Errored Seconds — a one-second period containing at least one bit error or defect — that the SONET/SDH Section 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 physical-layer bit errors on the SONET/SDH Section 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 csOc3SectionPrevDayESs value against the day before to see whether physical-layer bit errors on the SONET/SDH Section on the SONET/SDH Section layer of this OC3 port is trending better or worse.