csOc3SectionDayESs 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 so far today. 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, checking csOc3SectionDayESs mid-afternoon and finding it already well above a typical day's baseline is an early sign of physical-layer bit errors on the SONET/SDH Section on the SONET/SDH Section layer of this OC3 port, well before the day's tally is complete.