This counts correctable Header Check Sequence (HCS) errors on this ATM-over-SONET port since the last reset, single-bit errors in the cell header that the HEC algorithm was able to fix. A modest, steady count of correctable HCS errors on this specific port is generally tolerable since the error correction succeeded, but a rapidly rising rate is an early warning of degrading signal quality on this link before it worsens into uncorrectable errors. If pportHcsErrors on this port jumps sharply right as pportHcsSevereErrors also starts appearing, that combination signals the link's signal quality has crossed from marginal to actively dropping cells.