[IshKebab]

That was my first thought too. It's stupidly confusing to call something that acts nothing like a username a username. They clearly know that given the number of times they clarify how they work. Here's another:

> Note that a username is not the profile name that’s displayed in chats, it’s not a permanent handle, and not visible to the people you are chatting with in Signal. A username is simply a way to initiate contact on Signal without sharing your phone number.

[JoshTriplett]

It's absolutely a username. It can be changed arbitrarily whenever you like, and you'll probably in the future be able to have more than one name for the same underlying account, but it's still a username.

Other services do this too. For instance, you can sign up for some services with an email, and that's what you use to sign in, and you might be able to find other people by email if they let you, but you don't necessarily get shown someone's email on their profile, just the display name in their profile. And (in a well-designed service) you can change your email address at any time.

[XorNot]

Because a regular person, being given not a number for something, is going to call it a username.

Later explaining "you can have multiple usernames" is easier then trying to undo that conception. People are familiar with it. Your username is how you identify yourself on the computer in every context when it's not obviously your phone number.