This catches undervoltage on the transceiver's supply rail, which matters because a marginally low voltage can cause intermittent, hard-to-diagnose bit errors on the optical link well before the transceiver fails outright, so this alarm often provides the first real clue when troubleshooting seemingly random packet loss on a link. For example, a threshold of 3135 millivolts, about 5% below nominal, tripping would explain sporadic errors that a simple link-up/link-down check would otherwise miss.