FOX FLOAT CTD/DPS (9mm Shaft) Air Volume Tuning Kit

Changing volume spacers in the shock is an internal adjustment that allows you to change the amount of mid stroke and bottom out resistance.

If you have set your sag correctly and are using full travel (bottoming out) too easily, then you could install a larger spacer to increase bottom out resistance.
If you have set your sag correctly and are not using full travel, then you could install a smaller spacer to decrease bottom out resistance.