If you click on the 'sign up' link on this board, your username will be [username].sigils.yuku.com if you go to any other board besides this one, but if you click on the logo and sign up from www.yuku.com, then if the username you want is available, it will be just the username (no .sigils.yuku.com) .
If the username you want is not available, then come back here and sign up from Sygil, you will have the board extension, but as long as you are posting here it will not be seen (and you will get the username you want... sorta. I say sorta because there are certain usernames locally that are blocked so we don't have two people with the username Michael, for instance.)
Also keep in mind if you have a username I don't approve of (there's very few I don't approve of but if it is too close to someone else's or if it is offensive) I will add your username to a script that's on the board that will change the way it appears when you are here. It won't actually change your username, but the script is designed that whenever it sees a username on this board that I have in the script it will make it appear with something else that I choose. Your profile name and any other board you visit it will be the username you registered with.
On a similar note, if you'd like your username to display differently, let me know, I can change it with that script so here it will appear how you want it to here... for instance if you ended up having to put numbers in it (ie; joe2984793) I can take those out or even change it completely for you :)
If you sign up here, it doesn't matter, really, but my personal preference is to have everyone with a global username rather than a board extension after it. With a local (board extension) username, you can post at any board, they will just see the extension at the other board. It's sort of like free advertising for me. That's okay (I never mind advertising!) but I just like it better if the account is global :)