Your username does two jobs. It signs you in, and it gives you anything@yourname.skudo.org.

The subdomain

Any address on your subdomain works the first time somebody uses it. No creating in advance, and the monthly limit there is higher than on the bare shared domain because the subdomain carries less shared reputation risk.

Changing your username

Every account gets one username. You can change it any time from the Usernames page, but not as often as you like: the first change is free, and after that each change requires waiting longer than the one before (72 hours, then 7 days, then 14 days from then on, every time). This keeps a username from being churned quickly enough to make the permanent-block rule below meaningless.

Accounts that already held more than one username before this policy existed keep every one of them; nothing was taken away. If you need an exception, contact support.

Deleting one

Changing or deleting a username does not release it. Nobody else can register it afterwards, and neither can you.

This is deliberate. Addresses on that subdomain may still be written to for years, and handing the name to a stranger would hand them that mail. The cost is that names are finite, which is a cost we would rather carry than the alternative.

Choosing one

It appears in every address on that subdomain, and it is visible to everybody you give an address to. A username that is your real name makes every alias on it traceable to you by anybody who reads an address, which defeats a good part of the point.