This counts Severely Errored Framed Seconds on this OTU signal -- seconds where the receiver actually lost frame alignment on the OTN framing, not just detected bit errors within an otherwise-locked frame. Because this reading comes from a closed, completed 24-hour history bin for this OTU signal, losing frame lock, as opposed to just seeing bit errors within a locked frame, usually means the client traffic riding this OTU was briefly dropped outright rather than merely degraded. For example, an OTU port with 0 SEFS but a nonzero SES count had errors severe enough to count but never lost frame sync, whereas any nonzero SEFS on this signal correlates with a momentary traffic-affecting outage on that wavelength, across that completed 24-hour day.