csOc3SectionDaySEFSs is a read-only PerfCurrentCount counter reporting the number of Severely Errored Framing Seconds — a one-second period in which frame synchronization was lost at the Section layer — 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 a framing/clock mismatch between the two ends of the OC3 link 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 csOc3SectionDaySEFSs mid-afternoon and finding it already well above a typical day's baseline is an early sign of a framing/clock mismatch between the two ends of the OC3 link on the SONET/SDH Section layer of this OC3 port, well before the day's tally is complete.