The Preliminaries/The Tournament is basically the early rounds of the tournament to keep the amount of contestants low. Contestants are randomly chosen to fight one another until around half of them are out. The main tournament is just the same as the preliminaries except the competition is much tougher. Every ninja is randomly selected to fight another ninja. The victor will advance to the next round until there is one person left. Ultimately, the Jounins chose the Chunnin based on how they have performed.