This is the instantaneous, most-recently-sampled value of the transmitted optical power level measured at this port's output during the current performance-monitoring interval, expressed in the device's optical-power units. A live transmit-power reading tells an admin whether the laser is currently putting out the power it's supposed to, which is the first check when a far-end receiver starts reporting low incoming power. For example, if a newly installed transceiver reports outOpticalPowerInstant well below its datasheet value immediately after insertion, an admin would suspect a bad unit rather than wait to see a trend.