Q and BER are in the one-to-one matching relation. Usually, it is more convenient to use the Q value to present the system performance.

In the WDM system, when BER<1e-15, that Q value>18dB. So you can see over 18dB of FEC_BEF_COR_ER on U2000, that means BER is lower than 1e-15

You can see the mathematical expression for Q factor in the attached file.