Nintendo’s byzantine online account system just got even more complicated, with the addition of a new “User ID” to user accounts. The new User ID seems to be the way you’ll be identified online, similar to Microsoft’s Gamertag system. It is not the same as a Nickname, another form of identification used for the same accounts. You’ll need to log into your Nintendo Account here to claim a User ID, even if you already have a Nintendo Account set up.
If you’ve used an online Nintendo service before, chances are you’ve got at least some of this down, but here’s where things stand with Nintendo’s current accounts:
Nintendo Account
Your Nintendo Account is the top level of all your online Nintendo services. It’s attached to your email address, and can be accessed here. It’s the portal through which you’ll manage account settings, including your account nickname, your Nintendo Network ID, and your new Nintendo Account User ID.
Nintendo Network ID
Your Nintendo Network ID (not the same as your Nintendo Account User ID) was used for online services with the Wii U and 3DS, where it provided access to the Nintendo eShop and Miiverse. You can link your Nintendo Network ID to your Nintendo account through the Nintendo account settings page. While that allows you to share an eShop Wallet between consoles, downloaded games aren’t really tied to the account (something that appears to be changing with the Switch).