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One of the features available on the Y-Comm menu is the Surprise Trade. Select a Pokémon from your Boxes that you’re willing to trade, and that’s it! A trading partner will automatically be found as you continue on your adventure!
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These trades can be carried out with players from around the world if you’re using the internet. But with local communication, you can even get a trade from someone around you before you know what’s happening! Head out to lots of different places, and see if you can get a Surprise Trade! You won’t know what Pokémon you’ll get in exchange for your Pokémon until you actually receive it. If you’re lucky, you might end up with a rare Pokémon.
Membership in the Nintendo Switch Online (paid) service is required to participate in Link Trades and Link Battles using the internet. There’s no need to set up a membership for local communication use.