This PerfCounter64 counts Severely Errored Seconds encountered by a SONET/SDH VT Path in the history bin. Because SES on a VT path directly reflects the service quality of one individual sub-circuit, an admin uses it when a specific customer complains about a single DS1 while the rest of the STS-1 looks healthy. If vtVcPathHistorySESs shows a burst of severely errored seconds during the exact hour a customer reported garbled voice traffic on their DS1, that correlation confirms the VT path as the fault location.