This Gauge counts Severely Errored Seconds accumulated at the Line layer during the current 15-minute interval, the more serious Line-layer error metric analogous to Section-layer SES. An admin cares because a nonzero SES count at the Line layer during an interval where Section layer looks clean indicates a serious problem originating above the physical span, such as at an upstream add/drop multiplexer. For example, Line SES incrementing while Section SES stays at zero across the same interval points investigation toward line-terminating equipment rather than the fiber itself.