This read-only Counter64 reports how much the internal MAC receive errors count increased since the previous stats-polling interval for this interface's live, currently-active error-statistics record, i.e. the per-interval rate rather than the lifetime total. A nonzero value here means new failures of this specific kind are occurring right now on this link, not just accumulated history, 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. 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, which this delta field would show as a sudden nonzero reading in a single interval.