This reports whether this port's APS protection line selector is currently released (not in use) or selected (actively carrying traffic after a switchover). A selected status here on this specific port is a direct, real-time indication that a protection switchover has actually occurred and that this line, not the originally designated working line, is presently carrying live traffic, which matters for anyone assuming traffic is still on the primary path. Finding pportAPSprotectionLineState reporting selected during routine monitoring, without a corresponding known maintenance event, would indicate an unreported fault on the working line partner already triggered a live switchover.