Fediverse addresses have two parts, the username and the server name.
By default, Mastodon and most other Fedi server types hide the server part of the address within posts so that they are easier to read. All you will see in conversations is the first part of the address, so you would see @john even if the full address is @john@example.com. (This is similar to how real life conversations often just use a person’s first name.)
This means that it’s a good idea to choose something distinctive for a username, because in posts people will often just see the username on its own.
I’m not sure what you mean. Could you give some examples?
An account with the address @MarvellousWidgets@example.com, would look like @MarvellousWidgets in conversations. People can refer to it in discussions and it’s still clear who is being talked about: “Have you seen the latest release from @MarvellousWidgets?”
If it had a more generic username such as @software@example.com, all that people would see of its address in posts would be @software, and it’s not clear who is being talked about. A discussion using it would look like this: “Have you seen the latest release from @software?”. People would have to click on the username each time to find out who they are, because so many other accounts have a similar username.
I have my own server with a custom domain. That makes my address distinctive, right?
Yes and no. The full address with a custom domain is indeed very distinctive, but because the domain is hidden by default in discussions, it is still important to choose a distinctive username, as this is what most people see most of the time.
Don’t use dots / full stops / periods in usernames
Some Fediverse server types allow you to use . in usernames while others (including Mastodon) don’t allow it. People will not be able to follow you from Mastodon if you use a . in your username, so it’s safest just to avoid using . in usernames.
Don’t use really short usernames
If your username is very short, it is physically more difficult for people to click on it or tap it. Try to use at least four or five letters, preferably more.
What happens if two accounts with the same username are mentioned in the same post?
Mastodon etc will automatically show accounts’ full account addresses if two accounts with the same username are mentioned in the same post. This is also similar to real life conversations: if you were talking about two people called John in the same conversation, you might mention their surnames so that people know which John you’re referring to.
Why doesn’t Mastodon just show everyone’s full account addresses within posts?
Account addresses can be very long, depending on the domain name used by the account’s server. It can be quite hard to read a post where full addresses are used all the time. Just showing the username makes posts a lot more readable.
How do I find out the full account address of a username?
Just click on the username, that will take you to the user’s profile page which includes their full and unique account address.