how do I register a new iPad on my Apple account?
How do I register a used iPad to my Apple account?
iPad 2 Wi-Fi, iOS 10, not actually registered with Apple
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Apple doesn't register products for service/warranty purposes. They identify them by serial number. You can check coverage here: Check Your Service and Support Coverage - Apple Support
Also, devices running iOS 9 or later are automatically added to your Apple ID device list when you sign in to iCloud. See: Check your Apple ID device list to see where you're signed in - Apple Support