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 without touching the controller (when the demo normally would play, or just waiting for Mario to wake up after snoring), the game gets set to ACT 1, even though nothing happens. This is likely because the code for the vanilla demo is not fully cleared in B3313. ACT 1 can also be triggered by defeating the final boss and witnessing the ending cutscene. (Likely the game also goes into ACT 1 when playing the End Credits)

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

Places with things unlocked by ACT 1

 * Jolly Roger Lagoon: The Metal Cap Switch is only available on ACT 1 (the Metal Switch is normally found in Lethal Cavern on ACT 0.)
 * 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 it's 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 has a higher chance of sending you to Vanish Cap within the Plexus instead of Uncanny Moat, and vice versa for ACT 0.
 * Eternal Fort/Eternal Fort (beta): If ACT 1 is enabled, then killing Bowser will yield unique, extended dialogue.
 * Crimson Hallway: Dying with ACT 1 enabled will send you to Monochrome Castle Grounds, supposedly an intentional feature according to the lead developer. This only occurs if you're sent to Crimson Hallway via the Big Boo's Haunt warp in Plexal Lobby.
 * Hazardous Highway: The star in the stone platform section is exclusive to this mode.

The following two levels according to Chris were also meant to have differences in ACT 1, however, presumably due to an oversight, these changes only trigger after collecting 30 stars, instead of only appearing on ACT 1.


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

Trivia

 * On December 13th, 2021, Primarina discovered a 144th star on 0.7 in Tall, Tall Mountain with this feature.