ADN-MIB::adnX3TotalSESs records how many severely errored seconds (seconds with very high error rates) occurred in the past 24 hours, aggregated from 96 separate 15-minute measurements. Severe errors are more serious than ordinary errors and often indicate equipment failure or major environmental issues, so engineers prioritize this metric for alerting. A value of 0 is normal; a value of 2 or higher usually means the link needs immediate attention.