This read-only Unsigned64 gives the average per-interval delta of aggregate 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 this is the transmit-side counterpart to the receive-error rollup for this same interface, used as a first-pass check before looking at its deferred-transmission or internal-MAC-transmit detail. 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.