From the Super Mario Wiki
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Coin Edit is a mode exclusive to New Super Mario Bros. U. It appears only in the Coin Courses and is optional. Here, a player can place up to 300 Coins and up to three Star Coins almost freely by using the Wii U GamePad's stylus on the screen. There is a button that can make the player change sections, reset the layout, quit without saving or quit and save; by doing this, the player can store the layout being made. After this, the level can be played with the custom layout.
The player cannot place Coins everywhere though; some prohibited places are near an enemy, near a Pipe or near the level end and start. Also, the player cannot produce raining Coins or other special coin setups that are used in Nintendo-made courses.