Multitype changes the ability-bearer's type to match the Type Plate it is holding, granting STAB for moves of the corresponding type.
The moves Gastro Acid, Role Play, Skill Swap, Worry Seed, and the ability Trace do not affect a Pokémon with Multitype.
The moves Covet, Knock Off, Switcheroo, and Thief cannot remove Plates from a Pokémon with Multitype, though they can remove any other item.
Changes
In Generation 4, Covet, Knock Off, Switcheroo, and Thief do not affect Pokémon with Multitype at all.
