This is the raw per-collection-interval delta for frames received: it holds how much the underlying counter (the running total of FC frames this port has received since its counters were last reset (e.g. at a port or fabric interconnect reboot)) increased during the single most recent sampling interval, rather than the all-time total. For one specific historical error-statistics snapshot for an FC port, watching this delta rather than the lifetime counter is what lets monitoring catch a fresh burst of errors quickly instead of waiting for a slow-moving cumulative total to visibly move. For example, a delta of 6 in one interval on an otherwise-quiet port is a much stronger signal of an emerging problem than the same 6 buried inside a lifetime count of 50,000.