csDs3Ds1TotalSESs is a read-only PerfTotalCount counter reporting the number of Severely Errored Seconds — a one-second period with a very high error density on the DS1 — that this channelized DS1 (T1) circuit carried inside the DS3 has recorded summed across the entire previous 24-hour period, with any invalid 15-minute bucket counted as zero. A rising count here is the operational signal that a serious burst of errors on that particular T1 channel 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, comparing csDs3Ds1TotalSESs's 24-hour rollup from one night to the next lets an admin judge whether a serious burst of errors on that particular T1 channel on this channelized DS1 (T1) circuit carried inside the DS3 is a one-off blip or a worsening trend.