This read-only Counter64 reports how much the FCS (CRC) 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 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. For example, a steady trickle of a handful of FCS errors an hour might be tolerable, but thousands appearing in one interval on this port would justify swapping its transceiver first, which this delta field would show as a sudden nonzero reading in a single interval.