This is the lowest value recorded during the current performance-monitoring interval 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. This is the best moment of the interval and is expected to read zero on a healthy link, so even a very small non-zero minimum is worth investigating rather than dismissing. For example, if bitErrorRatePostFecMin is reported as exactly zero for every interval over a week, that's the confirmation an admin needs that FEC has had full margin the whole time.