Tip: Layer 3 backgrounds and tides disappear when you hit a message box, so plan your level around that. For example, you can add a sublevel without layer 3 at the start/midpoint, and include your message there.
This block disables Mario's jump and/or spinjump when Mario is touching it from above. It's pretty customisable, as it can be configured to only restrict Mario's jump on certain conditions (on/off switch, blue/silver p-switch and Mario is in water/starman/on Yoshi). Check the asm file for those customization options.