When you select an "All" option, everything will be matched to the hipfire sensitivity of the game you convert from. The only reason not to use this is if your aim preference is not achievable with the conversion methods available. It's highly recommended to use the "All" option however, as it makes conversion mathematically sane and consistent. People are different though, so some prefer other matches.
If you can't match your preference with the conversion methods, you need to do the calculations in (at least) two steps. First convert hipfire to hipfire, then convert ADS to ADS (or Scope to Scope) using the previously calculated hipfire sensitivity if the game uses that for ADS/Scope.
The separate aim selections are also useful for finding out the FOV of a specific aim, as this will be shown in the "Actual HFOV" and VFOV in the output.
No games other than some aim trainers give you the option to change FOV Type in the game. The reason for the option to change is that it also works as a converter. If you want the same FOV in Apex as in BFV, you simply change the FOV Type for Apex to Vdeg (because that's what BFV uses) and enter the exact same value as you have in BFV. The output will show you the Config FOV for Apex that you need to configure to match the FOV.