There are three versions. The S series which is the smallest. The Pro series which is intermediate in size, and the the X series. So every X is also a Pro. You will see the difference if you go to Garmins website and go to the link for the Fenix 6 series. When you select the different sizes the model names change. https://buy.garmin.com/en-US/US/p/641435/pn/010-02157-10. It can be a little confusing but you just need to decide what size you want and then the features.