This counter tallies seconds in which the T1 line's error rate was severe enough to cross the severely-errored-second threshold, a stricter condition than a plain errored second. A jump in SESL on this port, rather than in ESL alone, tells the admin the line isn't just occasionally glitching but bursting errors badly enough that traffic on that T1 is likely experiencing measurable degradation right now. For example, a SESL count that spikes from 0 to 40 in one polling cycle while ESL only rises modestly would suggest a sudden line fault, such as a repeater failure, worth an immediate site visit.