It used to be that registration opened and you just signed up online. As the tournament improved, it got to the point that it filled up in a matter of minutes, leaving out a lot of the best talent.

So they said if you placed x position or higher in y state, you can preregister. Now each state has a qualifier for the kids that don't meet the state placer preregistration criteria.

You can still just sign up online when registration opens, but you're taking a gamble on not getting in. I believe they also auction off a few spots for charity every year after registration closes.