This gamemode has the least restrictions, but is also the least "movie-realistic". There is no limit to how many players can play a certain class, and usually all supported classes and models should be available (dependent upon individual server settings). Open mode has a point progression system which allows you to upgrade your abilities as you play.
In Semi-Authentic (SA), the classes are limited to choosing the "skins" of the people who actually appeared within the time-period that the map takes place in. Players may still allocate points as they deem fit.
Full Authentic Mode
Full Authentic (FA) is very much different to both Open and Semi-Authentic modes. Not only are classes and models limited to match the current map, but also certain classes only allow a max number of players at any one time. If you want to play the class, but there are no spare slots, you will be put in a queue, and forced to play a different class until a slot is available to play the limited class. Also, class setups are fixed from the start of play. There is no point progression system available in Authentic mode.
In the most recent MovieBattles II builds (B19 and up), the traditional BaseJKA dueling mode has been replaced in favor of a Full Authentic mode of dueling, recreating well-known duels in the Star Wars universe, where you take control of a specific character involved in the duel, in the same place and time that the original duel occurred, and try to change history by defeating your opponent(s). Duels can have any number of participants, but are usually restricted to 1 vs 1, although there are a few instances of 3 vs 3 or 4 vs 1, or however many characters are needed to reenact the duel as it was meant to be.