This is the raw per-collection-interval delta for link failure events: it holds how much the underlying counter (counts how many times this port's physical FC link has gone down and had to reinitialize, i.e. loss-of-light or a failed link-level protocol negotiation) 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.