Dual-slot mode
The dongle method is one of the many ways that players of Pokémon Diamond and Pearl can obtain Pokémon that are outside of the Sinnoh Dex, especially Pokémon that are exclusive to other regions, by using the GBA games of the series as dongles for the DS games.
The majority of Pokémon that can be obtained in this way are in some way version-exclusive Pokémon to one of the Generation III games. With one of the five Game Boy Advance games inserted into the GBA slot of a Nintendo DS while playing either Diamond or Pearl, players will be able to find Pokémon not otherwise available in Generation IV in certain areas of Sinnoh, but only after they have unlocked the ability to use Pal Park as well.