This counts blocks of data on the radio link that arrived with errors severe enough that forward error correction could not repair them, meaning those blocks were effectively lost at the receiving end despite the link's error-correcting mechanism. It is a direct indicator of link quality degradation beyond what FEC can mask; a rising count here, even while the link stays technically up, points to fading, interference, or misalignment eating into the radio's error budget. A sudden jump in this counter during a rainstorm on a microwave hop is a classic sign of rain fade exceeding the link's FEC correction capacity.