S3 Stars Cylinder Hear, Outer part only.
The various inserts are available seperately on this website.
If you want fairly flat power for beginners or for really slippery conditions then you would set the bike up with a "low" compression insert. If you want to keep the bike standard, order the "standard" insert. If you wanted a more aggresive power setup, or you are going high in altitude then you could experiment with a "high" compression insert