This reports the ONU's internal supply voltage as read from its optical transceiver's digital diagnostics monitoring (DDM), typically a few volts. A voltage drifting outside spec often precedes an outright optical module failure well before the link actually drops, so this is a proactive-maintenance signal rather than a fault indicator on its own. For example, a nominal reading of '3.30V' slipping to '2.85V' over a few weeks flags a failing power regulator on that specific unit worth swapping before it fails in the field.