This read-only Unsigned64 holds the raw count of internal MAC receive errors -- errors this interface's own on-board MAC/ASIC logged while receiving frames, as distinct from wire-level CRC or alignment problems -- recorded in this one archived historical sample of an interface's error-statistics. 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. For example, if this grows on a port while its FCS counter stays flat, the adapter hardware behind this port -- not the cabling -- is the suspect.