This is the instantaneous, most-recently-sampled value of the residual bit error ratio measured after forward error correction has attempted to fix the errors detected on the incoming signal, expressed as a bit-ratio. Because FEC is designed to drive this down to effectively zero under normal conditions, any non-zero instantaneous reading right now means FEC is currently failing to fully correct the signal, which is a much more serious sign than a pre-FEC blip. For example, if bitErrorRatePostFecInstant reads non-zero while the pre-FEC counter is only moderately elevated, that combination tells an admin the FEC engine itself, not just the fiber, needs attention.