SR button

The SR button refers to the gray right shoulder button on both individual Joy-Cons when it is detached from the Nintendo Switch. The wrist strap that comes with the Joy-Con can be attached to make the button easier to press.

Overview
When using dual Joy-Cons, it serves no purpose and is completely inaccessible when the Switch is in Handheld mode. When using a single Joy-Con, it serves as an equivalent to the R button, acting as a shoulder trigger. In Ultimate, it is used for shielding, much like how the R button is used for shielding in.