This read-only Unsigned64 gives the average per-interval delta of FCS (CRC) 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 this is the textbook signature of a degrading SFP, a marginal cable run, or electrical interference specifically on this link, as opposed to a congestion-related counter. 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.