This OID counts, in seconds, how many one-second intervals on this ODU (Optical Data Unit) layer of this optical transport interface were severely errored, meaning the error rate in that second crossed a much higher threshold than an ordinary errored second. Severely errored seconds are weighted more heavily than plain errored seconds in most SLA calculations, since they represent intervals bad enough to likely cause visible service disruption rather than just a few corrected bit errors. For example, a burst of severely errored seconds on the ODU layer beneath an OTU4 wavelength carrying aggregated client traffic coinciding with a weather event would tell an operator the fault is likely an environmental fiber issue rather than routine noise.