On fluxer.com, every username has a four-digit tag after it, like alex#4821, and changing your username gives you a new random tag unless you have Plutonium.
The four-digit number is your tag, and it lets many people share a username. Usernames aren't case sensitive, so Mina#4821 and mina#4821 count as the same username.
How do I change my username?
In the web and desktop apps, open "User settings", go to "Profile" and click "Change username". You need to confirm your email address before you can change it.
If you change your username many times in a short time, Fluxer asks you to wait.
Why did my tag change?
Without Plutonium, changing your username gives you a new random tag. Changing only the capital letters in your username keeps the tag you have.
If you picked your tag or changed your username while you had Plutonium, your tag changes to a random one when your Plutonium ends, unless it renews. The app warns you before you save a new username, and the Plutonium page's footnote says the same.
Can I choose my tag?
Yes, if you have Plutonium. You can pick the tag you want, or keep your current tag when you change your username, as long as nobody else with the same username already has it. The #0000 tag is reserved for Visionary members and can't be picked.