You can have either one of two Apple watches. One is used with your Iphone, has a bluetooth connection to your watch, and will not work as an Internet/cellular device if not within range of your Iphone. The other is a device with it's own modem so to speak and will work with all cell services without your Iphone when out of range of your phone.
If you purchased your Apple watch through Verizon, you for sure have one that requires it's own line in order to work independently of you Iphone. Verizon does have a $10 device charge.
Hope that helps.
boyc, we always want you to understand your charges fully. When you add a device to your account, it will be assigned a mobile number and line access charge. In your case, your smartwatch is set up on its own line which is why you are seeing this mobile number attached to it. A smartwatch line is charged $10.00 for the cellular service/plan.
Smartwatches also use Numbershare. This allows your smartphone and smartwatch to run off of your same number (smartphone number). This link goes over this further: https://www.verizonwireless.com/support/numbershare-faqs/#numbershare
CassieG_VZW $10.00 is the cost of the line, and for the number share feature that allows the watch's line to use the phone's mobile number as it's own. This provides the watch with cellphone service, and the use of the cell phones mobile number. Without the line, the watch would use Bluetooth.