Each time you StreetPass another Pokemon player or trade via the GTS and Wonder Trade, Poke Miles are added to your account. The amount of Poke Miles depends on the location that each 3DS is registered to (trading worldwide earns you more while trading locally earns you less). Poke Miles can be spent in the Pokemon Center in Lumiouse City. Inside, the old man in the white suit offers a variety of items in exchange for Poke Miles.
You can also buy the following items for the corresponding Poke Miles at the Global Link Mileage Club for up to 50% off after registering your game: