Who would have thought that Nintendo having different account systems would be so confusing for people? It's December 2017, and in Japan, Nintendo says it's getting lots of questions about the differences.
— 任天堂株式会社 (@Nintendo) December 25, 2017
"Currently, we are getting a great number of inquiries about the Nintendo Account," the company wrote in a tweet. The attached image easily shows the difference between the Nintendo Account (for smart devices, computers and Switch consoles) and the Nintendo Network ID (for the 3DS series and the Wii U).
But why all the questions now? The Switch is becoming more available in Japan, so there are, no doubt, new owners confused by the new account system.
Hopefully this clears up any confusion!