What is a sign in ID?


A ♥Bill Cipher♥ fan
On my nintendo switch when i wanna link a nintendo account it says sign in with email OR sign in ID, how do i get a sign in ID, and do i have to still have an email address? i wanna know because i don't have a email address.
the sign-in ID is a username which i believe you can set in your settings somewhere after youve already made an account/maybe when you're setting it up, and you can use that for sign-in after its been set
you would still need an email address to register an account, just like basically every account system to ever exist (including the previous Nintendo Network ID system and the one on this very forum)
That's funny cause i could still link a NNID to my wii u without having to use a email address, but for my wii u i had to make up a email address......cause i didn't have a resl one....but it still let me link a ID. Why is that?