From the website:

The Purina® Incredible Dog Challenge® is an invitational event, featuring some of the best dogs in their sport. We generally scout these dogs through various sanctioned organizations, local clubs, and other national competitions.

Local dogs, however, do have a chance to earn a right to compete in the sport of flying disc and diving dog along with the invited competitors for each show. The top two finishers in each qualifying event will be seeded with the invited competitors for Saturday’s competition.