This counts the individual bit or symbol errors that the OTU's Forward Error Correction actually corrected during the bin. Because this reading comes from a closed, completed 24-hour history bin for this OTU signal, FEC corrects errors silently, so this count on this specific signal is often the only visible sign the underlying optical path is degrading even while the corrected, error-free signal keeps passing traffic normally. For example, a FEC correction count climbing steadily bin-over-bin on this OTU while the client signal shows zero errors is the classic 'FEC is masking a real problem' pattern worth escalating before the correction margin runs out, across that completed 24-hour day.