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 the still-filling, live 15-minute 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, so far within this still-open 15-minute interval.