This counter reports the number of Severely Errored Seconds encountered at the Section layer during one specific historical 15-minute interval in the past 24 hours. An admin cares because a historical SES spike (versus plain ES) at a particular interval flags a moment of serious, not just minor, physical degradation, useful when correlating a past outage report against exactly which 15-minute window it occurred in. For example, a customer-reported brief outage two hours ago could be confirmed by finding a nonzero SES count in the interval corresponding to that time.