The R button refers to the right shoulder button on the Wii U GamePad, the Nintendo 64, GameCube, Classic, and Wii U Pro Controllers, and on the Nintendo 3DS itself. On the GameCube, Classic, Wii U Pro controller, Wii U GamePad and Nintendo 3DS, it is used for shielding, rolling, air dodging in midair, and sidestep dodging, which is shared with the L button in all of these controllers (aside from 3DS where L is grab) while on the Nintendo 64 controller, dual Joycons and Switch Pro controller, it is used for grabbing.
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, SR is used for shielding and SL for grabbing. In this configuration, the L, R, ZL, and ZR buttons cannot be set to grab.