This Gauge counts Errored Seconds accumulated at the Line layer during the current 15-minute interval, the Line-layer counterpart to the Section-layer ES counter, covering the SONET span between multiplexers rather than the lower Section span. An admin cares because Line-layer ES errors that don't correlate with Section-layer ES errors suggest the problem is specific to line-overhead processing or occurs further upstream, not on this local physical span. For example, a rising Line ES count with a clean Section ES count points toward an issue with line overhead handling or an upstream regenerator, not this segment's own optics.