From SmashWiki, the Super Smash Bros. wiki
Smash Controller is a companion app for Super Smash Bros. 4, available since June 14, 2015 on the Nintendo 3DS eShop for $0.99 USD. The app is a standalone instance of the "Wii U" menu in Super Smash Bros. for Nintendo 3DS, allowing players without their own copy of the game to use their 3DS system as a controller in Super Smash Bros. for Wii U. This is the only functionality of the app; the ability to transfer custom characters between systems is disabled, and the connection display on the Wii U screen will indicate that the 3DS system is connecting through the app rather than the full game by displaying its Home Menu icon instead of the 3DS version logo. Additionally, the controller port number and chosen character do not show up on the top screen as they do in Smash 3DS; instead, the title screen of the app is shown the entire time the app is open.