This counts, at the line layer, severely errored seconds, seconds with a high enough error density to signal serious degradation rather than an isolated bit error on this T3/E3 interface during the current 15-minute performance-monitoring interval, following the ANSI T1.231 performance-monitoring parameter set. It resets every 15 minutes, so an administrator polling it mid-interval sees a still-accumulating count for the current window, and an administrator watching it alongside its Path- or Line-layer sibling counters gets a standard, carrier-comparable picture of the circuit's error performance for that window.