csOc3LineDayCVs is a read-only PerfCurrentCount counter reporting the number of Coding Violations — individual B3ZS/frame bit-parity errors on the SONET/SDH signal — 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 marginal optical signal affecting the line 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 csOc3LineDayCVs mid-afternoon and finding it already well above a typical day's baseline is an early sign of a marginal optical signal affecting the line on the SONET/SDH Line layer of this OC3 port, well before the day's tally is complete.