Rockshox Deluxe/Super Deluxe "Gnar Dog" Volume Reducer Kit 2017-22

Rockshox Deluxe/Super Deluxe "Gnar Dog" Volume Spacers 2017-22

Volume Spacers reduce the volume of the air chamber. This makes the shock more progressive, helping with bottoming out.

Contrary to what you'll find on various websites, adding spacers isn't a good solution to stop diving, it just makes the shock harder, deeper in the travel. If you fit them as a way to stop diving, you will most likely never use all the available travel.