ACT 1

ACT 1 is a mechanic used in B3313 to alter and change the game in subtle ways.

Explanation
In B3313, you normally play on ACT 0. this is due to the fact that unlike vanilla, B3313 does not have a Star / Act selection screen. However, if you were to wait around 35 seconds on the title screen (when Mario says "no" whilst snoring), the game gets set to ACT 1, even though no demo actually plays on the title screen. This is likely just the code for the demo playing wasn't fully wiped in B3313. ACT 1 can also be triggered by defeating the final boss and witnessing the ending cutscene.

When you activate ACT 1, There's no way to go back to ACT 0 except for resetting the game.

Places that have stuff unlocked by ACT 1
The following two levels are according to Chris also meant to have differences in ACT 1, however presumably due to an oversight, these changes only trigger after collecting 30 stars, and appear regardless of whether or not the player is in ACT 1.
 * Jolly Roger Lagoon: The Metal Cap Switch is only available on ACT 1.
 * Grassy Snowy Summit: 1 star can be found within the Grassy state inside an alcove where the waterfall originally was.
 * Tall, Tall Mountain (beta): 1 star can be found at the top of the waterfall.
 * Jolly Roger Bay (beta): The Ghost Ship and its star only appears after triggering ACT 1, then standing on the bow of the regular ship after it is raised.
 * Silent Hall: If ACT 1 is activated, the Vanish Cap painting is more likely to send you to Vanish Cap within the Plexus instead of the other way around.
 * Eternal Fort/Eternal Fort (beta): If ACT 1 is enabled, then killing Bowser will yield unique, extended dialogue.


 * Bob-omb Battlefield (beta): The tower with a star on it is only meant to appear in ACT 1.
 * Whomp's Kingdom: According to Chris, the SMG2 island that appears is meant to only show up in ACT 1.