An Exp. All is an item that allows all party Pokémon to gain experience from a battle, including Pokémon that do not participate in the battle. It can be turned on or off by using it; if turned on, an Exp. All will take effect as long as it is in the player's inventory.
The Exp. All will award 50% experience and the full amount of EVs to Pokémon that do not participate in battle. Pokémon that participate in battle are unaffected by the Exp. All, gaining the full amount of experience and EVs as normal. Having multiple Exp. Alls that are turned on will grant the same effect as having a single enabled Exp. All.