This counter tracks Severely Errored Seconds — seconds with a much higher density of Section-layer coding violations than a plain Errored Second, or containing certain major defects — accumulated during the current 15-minute interval. An admin cares because SES is a stricter, more serious threshold than plain ES, and a nonzero SES count at the Section layer is a stronger signal of a real physical fault (versus occasional noise) than ES alone. For example, an interval showing several ES but zero SES suggests scattered minor errors, while any nonzero SES count in the same interval indicates a more serious burst of degradation.