If used from the Bag, it changes a Pokémon's Ability from one of its standard Abilities to its Hidden Ability. It cannot be used to change a Pokémon's Ability from its Hidden Ability to one of its standard Abilities.
The Ability Patch can only be used on a Pokémon that belongs to a species that has a Hidden Ability. For example, it can be used to change a Rookidee's Ability from Keen Eye or Unnerve to Big Pecks, but it cannot be used to change a Metapod's Ability (which is always Shed Skin).
A Pokémon that has its Hidden Ability due to the Ability Patch can pass down its Hidden Ability via breeding like any other Pokémon with its Hidden Ability.
The Ability change is retained even if the Pokémon evolves, and the Ability slot provided by the Ability Patch is maintained if the evolved form has a different standard Ability. For example, a Lillipup with the Abilities Vital Spirit or Pickup evolves into a Herdier with the Abilities Intimidate or Sand Rush, respectively, but if one of Lillipup's two Abilities is changed to Run Away with an Ability Patch, it will have the Ability Scrappy after evolution.
||A patch that allows a Pokémon with a regular Ability to have a rare Ability.