This read-only Unsigned64 gives the average per-interval delta of internal MAC receive errors across all the sampling intervals collected so far recorded in this one archived historical sample of an interface's error-statistics, 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 a rising count here implicates the local silicon on this specific adapter or IOM port rather than the cable, which is exactly why UCS breaks this counter out separately from the wire-level ones. 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.