This read-only Unsigned64 gives the average per-interval delta of internal MAC transmit errors across all the sampling intervals collected so far for this interface's live, currently-active error-statistics record, smoothing out interval-to-interval noise. Admins trend this average over time on a given link to tell a persistent, low-grade problem from a one-off blip, 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. An average that creeps upward release over release on the same port, even while individual deltas look small, is the kind of slow drift that justifies scheduling a cable or optic replacement.