This read-only Unsigned64 holds the raw count of internal MAC transmit errors -- errors this interface's own on-board MAC/ASIC logged while transmitting frames out onto the wire -- recorded in this one archived historical sample of an interface's error-statistics. Because this is a transmit-side hardware fault rather than a wire issue, a nonzero value here tends to implicate the local card behind this specific port rather than anything downstream. For example, an admin chasing one-way traffic loss on this link would check this counter to see whether the local adapter itself is failing to get frames out.