Old Selection Algorithm
Old Selection Algorithm is a temporary sidemod that restores 7.0.7's spawning logic to that seen in earlier versions. Without this mod installed, common Pokémon will be less common and rarer Pokémon will be more common. This is due to a bug that was fixed, but that the old spawn files were balanced around.
Old Selection Algorithm can be used both in single player and on servers. It is not required by the client when connecting to a server.