R button

The R button refers to the right shoulder button on the Nintendo 64 controller, GameCube controller, and the Nintendo 3DS, as well as one of two right shoulder buttons on the Classic Controller, Wii U GamePad, Wii U Pro Controller, New Nintendo 3DS, right Joy-Con and Nintendo Switch Pro Controller.

Overview
The functionality of the R button has changed throughout the series. On the N64 controller, dual Joy-Cons and Switch Pro controller, it is used for grabbing. On the GameCube, Classic, Wii U GamePad, Wii U Pro controller, and Nintendo 3DS, it is used for shielding, rolling, air dodging, and spot dodging, which is shared with the L button in all of these controllers (aside from 3DS where L is grab). Outside of gameplay, the R button is used to navigate some menus like tips and move lists.

When using a single Joy-Con in Ultimate, the R or L button (depending on which Joy-Con is used) is not set to any action by default. Instead, the SR button takes its place is used for shielding. In this configuration, the L, R, ZL and ZR buttons cannot be set to grab.

In Super Smash Bros. Brawl,, , and Super Smash Bros. Ultimate, the R button can be customized in the controls menu to perform other functions. It should be noted that the 3DS version has the ZL button perform the same functions as the R button due to the game releasing before the New Nintendo 3DS introduced a ZL button to the system. Prior to version 1.0.5 of the game, these buttons' functions were reversed, so ZR would trigger the same actions as R, though their functions outside of battle (such as in menus) were the same as in current versions. Nintendo Switch hardware version 10.0.0 introduced the ability to map the functionality of any button to any other button from the console options menu, further diversifying how the R button can be used in Ultimate.

Trivia

 * Masahiro Sakurai had proposed the L and R buttons on the GameCube controller be replaced with a scroll wheel, as he believes that would be the ideal method of menu navigation.