This is the raw per-collection-interval delta for undersized (runt) frames received: it holds how much the underlying counter (counts inbound FC frames that were shorter than the minimum legal FC frame size and were rejected) 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.