Tip: You can use Lunar Magic's Overworld Layer 3 Editing system to edit or create your own layer 3 images (castle bricks, fish, rocks) and apply unique effects per level via the Layer 3 Bypass in the main editor.
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.