The higher the setting, the more the detector tries to call questionable targets Iron. This can help in eliminating iron trash targets from being dug. Everything is a trade off - The higher the setting, the more likely the detector will call a masked good target "iron" if the good target is near a piece of iron (or nail). In an iron infested site you can raise the I/B so you don't dig too much trash but you run the risk of missing those co-located masked targets. The Original Nox 800 had an Iron Bias titled FE with a setting of 0 -8. Minelab came out with a software update the included adding the F2 settings or adjustment. They did this while keeping the original FE so you can toggle between FE and F2. The F2 goes even lower than the FE but they kept to same 0 -8 adjustment numbers, but the scale between FE and F2 is different. A setting of 4 on the F2 scale is the same as the FE setting of 0. so you can see that the new F2 has Four settings that are lower than FE - 0, and those are F2 settings 3,2,1 &, 0. Those new to the NOX 800 should probably start out with an Iron Bias setting of about 4 on the FE scale or about 6 on the F2 scale until they find out how much iron they're digging. Then you can increase or decrease the setting as you become more familiar with the detector's behavior. The ideal would be to eventually work your way down to F2 - 0 as you become accustomed to the Nox audio. This will achieve maximum unmasking abilities.
I'm sure others will chime in with possibly a better way to explain the difference between FE and F2. Hope this helps.