This sets the exponent N (3 to 5) defining the Signal Failure BER threshold for this port's APS protection; a line bit-error rate worse than 10^-N triggers an SF failure and a protection switchover, which then clears once the BER improves to better than 10^-7. Because this triggers an actual APS switchover, tuning this threshold on this specific port controls how tolerant the line is of degraded quality before automatically failing over to the protection line; a lower N, like 3, declares failure sooner at a coarser error rate, while 5 waits for a more severely degraded signal. Setting pportAPSsfBerThresh to 4 on this port means a sustained bit-error rate worse than 1 in 10,000 triggers an automatic switch to the protection line, which then will not clear back until the BER improves past 1 in 10 million.