What is this game jam subdomain?

When we register to backtrace, do we insert this subdomain or we create a personnal one?

Put in whatever you want.  You can think of it almost like the name of your backtrace instance.  When you sign up, Backtrace creates an instance for you and uses the subdomain you input to do so.  Note that your subdomain shouldn't have any "." characters in it!

All right , thank you!