Tip: Switch palace switches normally disappear if you replay the level. However, if you insert them as direct Map16 and use the hex-edit at $00EEB2 in the ROM map, you can replay the level without a problem.
Also previously known as 'Floor Generator', this makes it so the player cannot move horizontally when offscreen from below, which is useful for people wanting to prevent players from skipping parts of a level from below. You can also set it to hurt or kill the player through a define.
If you want to prevent the player going under/above objects placed at the top and bottom of the level without moving the bottomless pit higher up/using a ceiling generator, use this patch instead. This extends the “hitbox” of the blocks out the level by constraining (or clamping) the hitbox of sprites and Mario to the edges of the level.