Hdeg 4:3 is the FOV type the game uses, the calculator will always default to the correct FOV type for the game you select. When you switch to Vdeg, you simply tell the calculator that you want for instance 72 vertical degrees. It will then calculate what you actually need to configure in the game to get exactly this.
Absolutely, the output will then tell you what multiplier you need to use to get the Hdeg 4:3 FOV you want.
Any of the FOV types work for this, but the output doesn't tell you all of them. It only shows you the true vdeg and hdeg, so it's easiest to use one of them. Vdeg is also the most commonly used type. If you already know your preferred FOV, let's say 90 Hdeg 4:3, you can perfectly fine use this for all games that supports changing FOV.