From the Super Mario Wiki
Ads keep the MarioWiki independent and free :)
Regional coins are one of the main collectibles in Super Mario Odyssey and differ from regular coins that are used as a global currency instead. They are purple and their shape and design vary in each kingdom. They reappear in Super Smash Bros. Ultimate.
Super Mario Odyssey
Regional coins first appear in Super Mario Odyssey. They can only be used in their corresponding kingdoms to buy outfits, stickers and souvenirs in the Crazy Cap stores. There are 50 or 100 located in most kingdoms, and their shape and design vary in each kingdom. After being collected, they are replaced with clear replicas which are worth 2 regular coins. The Cloud Kingdom, Ruined Kingdom, Dark Side, and Darker Side do not have regional coins. When a regional coin is collected for the first time, it makes a sound that resembles the combination of gold coins and Star Bits.
Super Smash Bros. Ultimate
Regional coins reappear in Super Smash Bros. Ultimate where the regional coins from the Metro Kingdom, as well as the aforementioned sound effect heard when a new regional coin is collected, can occasionally be seen during Mario's Super Jump Punch in place of regular coins from Super Mario 64 or Star Coins from the New Super Mario Bros. series of games.
Names in other languages