This counts frames on this interface that were deferred for longer than the normal allowed limit before they could be transmitted. Excessive deferral is a step beyond the ordinary TxDeferPkts wait -- it means this port's transmit attempts were held back so long during the current, still-accumulating short interval bin that the firmware logs it as an error condition, a sign of sustained medium contention or congestion rather than a brief, normal wait. For example, TxExDeferPkts moving off zero for this in-progress short-interval bin on a port already showing elevated TxDeferPkts would confirm the contention on that link has crossed from occasional to chronic.