This sleeve is meant to slide over the focus ring on the Fuji X100 and X100S. It gives you a larger surface to grip when you are adjusting the manual focus of the camera.
The focus tab is similar to those found on some Leica lenses and helps make manual focusing even quicker.
There is also a version that has the focus tab removed. For that model, go here: https://www.shapeways.com/model/1382512/fuji-x100-focus-ring-sleeve.html?li=my-models&materialId=25
NOTE: Due to variability in the finishing process, a small piece of tape may be required on the inside of the ring to ensure a tight fit.