Nintendo has a tumultuous history with online functionality, and a new addition to its already more complicated than necessary account system has quietly appeared. As the Switch is nearly here, this new feature is surely involved with how online interactivity operates on the new console.
This is not a separate feature, but what appears to be another form of identification added to the overall My Nintendo account. In addition to the Nintendo Network IDs, which you use for the Wii U and 3DS, users will now need to add a User ID. This is a identifier separate from the Nintendo account or Nintendo Network ID.
It sounds like perhaps this User ID will serve as a more public-facing identifier for players, or it could have something to do with how ownership is reflected on the Switch. Who knows? Nintendo has been conspicuously quiet about everything, and this is merely the latest chapter. In the meantime, it's probably wise to go claim a User ID for yourself before the Switch launches!