This counts FEC blocks on this OTU signal that arrived with more errors than the FEC algorithm could correct, meaning those blocks passed errors through uncorrected. Because this reading sits in the rolling window of completed 15-minute history bins for this OTU signal, this is the metric that most directly predicts actual client-traffic errors, since every uncorrected block here represents data FEC failed to fully repair on this signal, unlike the plain FEC-correction count, which only reflects successfully corrected errors. For example, an OTU signal reporting zero uncorrected FEC blocks even during a bin with a high correction count means FEC is still keeping up; the moment this count on that same signal goes nonzero, client traffic is actually seeing bit errors, within that completed 15-minute window.