A count of Severely Errored Seconds — seconds whose error or defect rate was high enough to be classed as severely degraded rather than just errored — recorded under the vendor-defined ('User') supplemental counter set, accumulated over the full lifetime of the interface, i.e. since the system was last powered on or the counter was last reset. Because SES weighs much worse than an ordinary errored second, an admin treats a nonzero count here as a stronger signal of real circuit trouble than the plain ES counter.