If you purchase The Hidden Treasure of Area Zero DLC*, you’ll be able to receive a New Uniform Set. You’ll be able to wear this outfit in the Pokémon™ Scarlet game, and also in The Hidden Treasure of Area Zero DLC once it’s released. The New Uniform Set includes uniforms for spring, summer, autumn, and winter. Please note that the appearance of the New Uniform Set will differ between the Pokémon Scarlet and Pokémon Violet games.

After purchasing The Hidden Treasure of Area Zero, you can claim your apparel items by opening the X menu, then selecting the Downloadable Content banner button at the bottom right of the menu. A Nintendo Switch Online membership is not required.

More details on the official game page of New Uniform Set ...