But if you select an item in a menu and don't want to use it, press B to go back.

That is to say, once you select an attack in battle, you can't press B and go back - it's too late.

This only works if your selection didn't make something happen. In the various in-game and battle menus, if you select something and you don't to commit to that selection, press B to go back to the previous menu. The B Button is known as the 'cancel' or 'back' button. When you do that, press Select at any point out of battle to use said item. It has a few in-game functions, but it's most important in-game function is when you set an item to the Select Button. The Start Button's primary function is to call up the main menu anytime when you're not in battle. That is to say the D-Pad allows you to move your character around various areas, whether it be a town, city, forest or cave, but it also allows you to navigate the various menus, both of the in-game and battle variety. The Directional Pad (or D-Pad for short) is your navigation button.

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