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.
||A patch that allows a Pokémon with a regular Ability to have a rare Ability.