Yes it is $10/month added to your cell provider’s plan because it is considered another device (like a tablet with Wi-Fi) that is independent of your phone, sort of. If you purchase the GPS + CELLULAR, it is initially synced up to your iPhone, and a new phone number(*) is assigned to it, and then once you activate the data plan (easy to do too, I did it via watch and iPhone while I was syncing it), you can leave your iPhone at home and just have your watch (let’s say if you are going to the market, hiking, jogging, swimming, or some type of activity). Also, the new phone number(*) is not one that you will use, it is merely assigned so that your iPhone can send info/signals to your watch (for the Cellular version), so your iPhone number remains still the one you will use. I found this out today when I checked my Verizon account, I had a new number assigned on my Apple Watch. And you can deactivate the data plan anytime if you feel you no longer need it and then your watch would just function like the regular GPS only version.