A recently announced feature for Pokémon X and Y, Pokémon Bank will allow players to save up to 3000 of their Pokémon to a Nintendo-sponsored cloud storage, for a small fee of $4.99 per year. The app functions for both fifth and sixth generation titles—Black, White, Black 2, White 2, X, and Y—allowing users to transfer Pokémon from a fifth generation cartridge to one of the sixth.
However, for players looking to use that last feature, there is a catch. According to the
Pokémon Bank FAQ, once a fifth generation Pokémon has been sent to the cloud, it can only be outputted to a sixth generation game, not back to the fifth generation game from which it originally came.
“Poké Transporter is a free downloadable application for the Nintendo 3DS system that will be available along with Pokémon Bank. Note that Poké Transporter cannot return Pokémon to Pokémon Black Version, Pokémon White Version, Pokémon Black Version 2, or Pokémon White Version 2 after they have been stored in an online Box.”
Source: Pokémon Bank FAQ