A:Answer If you purchase the watch with only gps, you do not need a data plan and are able to see your messages and emails as long as your Apple phone is in close proximity
If you purchase the gps/ cellular, it will do the same as above but if you want to use the watch without your Apple phone nearby, you need a data plan and then the watch functions independently and you are also able to use as a “phone”. I have ATT and the cost is an additional $10/month for cellular service
Hope this helps ...