A Pokémon with Multitype will change its type to correspond with whatever Plate it is holding; if it holds no Plate, the type defaults to Normal. Mutlitype also prevents any item loss through attacks such as Covet, Knock Off, Switcheroo, Thief and Trick. Multitype is not affected by Skill Swap or the Trace Ability.
Multitype doesn't have an effect in the Overworld, but the held plate can be changed outside of battle.
Pokémon With Multitype