This is the core game and one of the modes you'll probably play often. In this mode, you can play against up to four players (AI or humans). You can freely edit the match Rules as you see fit if you're playing Solo or the host of a local multiplayer game. This mode allows you to fight using your created Mii Fighter and you can also toggle the settings to allow customized characters from the main roster.
In this mode, you can also unlock new characters.
Upon accessing the Smash menu, you'll see the following options: