Make sure to take them into consideration when making tilesets from editing those Graphics.
It's worth noting that the bridge under Bowser and the chain holding it had a hidden tile property in Mari0 1.6 and below, but it was unhidden for Mari0: SE. You also couldn't edit the SMB tileset until SE, so applying it to other tiles wasn't possible.
The only tile with this property is Tile #1, which serves as a blank tile. Despite having a purple checkerboard appearance while selecting it, it shows nothing when placed. The only purpose of reskinning it would be for the amusement of the user.
A Used Block appears when Mario empties a breakable block or a Coin Block of its contents. The Used Block shown varies by palette and the item that was in the block. There are quite a few Used Blocks, and each one is hardcoded into the game, referenced by its position in the SMB tileset. They change with the spriteset.
Using Custom Graphics, one can modify the Used Block to produce various effects. Of note is setting its property to Breakable or Coinblock, allowing the player to hit the block indefinitely for as many items as they want.
The only used 2×2 block in the game is Tile #136, which is the giant portal tile's top right piece by default. It is a shortcut for placing the entire 2×2 block. Using it places Tile #137 to the right of Tile #136, Tile #138 directly under Tile #136, and Tile #139 to the left of Tile #138.