Fatigue rating confusion

I don't think most people understand the Don't Rel b4 rating settings mean that you if you set the rating at F0, unless your pitcher is struggling, he won't be relieved till he is almost spent. F9 means he is fully charged and F0 means there is not much left in the tank. You normally don't want to set you F? high, or the computer might pull that pitcher faster than you might like