This counter reports the number of line-level Severely Errored Seconds on this T1/DS1 port, as stated in the MIB description. A Severely Errored Second is a stricter threshold than a plain Errored Second -- it means the error rate in that one-second interval was bad enough to be classified as a major service-affecting event, not just a minor blip -- so a nonzero count here is a much stronger signal of an active, ongoing line problem than a nonzero Esl count alone. For example, 2 severely errored seconds appearing on this T1 port in one bin, even if the overall Esl count is modest, is enough on its own to justify dispatching a technician to check the physical line.