This read-only Unsigned64 reports how much the internal MAC transmit errors count increased since the previous stats-polling interval recorded in this one archived historical sample of an interface's error-statistics, i.e. the per-interval rate rather than the lifetime total. A nonzero value here means new failures of this specific kind are occurring right now on this link, not just accumulated history, since 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, which this delta field would show as a sudden nonzero reading in a single interval.