Each year, the Science Olympiad national organization publishes a Rules Manual for each secondary school Division (B and C) which contains the official rules for each of the 23 national events.
It is common for either the event slate or the rules for specific events to vary between different parts of the nation - most commonly different states, but also individual tournaments.
Frequently, state tournaments - and sometimes regional and invitational tournaments - run trial events. These are events not counted toward the team score. Usually these are events that are being tested in hopes/preparation of eventually being run as national events (see the national organization's trial events page), although sometimes they are just run for fun. In many cases alternates are allowed to compete in trial events.
State-Specific Event Slate
Several states run a modified slate of events. The following is a list of those states which do so. Note that this is not a comprehensive list.