It means that the actual FOV number you see in Apex is "assuming" you have a 4:3 resolution. So if you set it to say 90, it will be 90 for 4:3, but for 16:9 it will be 106.26 because it adds on the extra width.
As for converting from R6, it depends on your aiming preference for R6. ADS and ACOG scales differently (meaning you can't match them in a sensible way), so which one do you prefer if you have to choose one?