This is a term I developed to describe a certain subset of Pokémon games. These are games that can bi-directionally communicate with the core series games, but for one reason or another are not often considered to be core series games by the majority of Pokémon fans - whether because of different mechanics, a different console, or lacking some/all of the standard elements of the core series games.
In my definition, this encompasses the Stadium series, Colosseum, XD, Battle Revolution, and Legends: Arceus. Pokémon Go may qualify since it is compatible with Home and some species/forms can only be obtained in Go; I simply don't know enough about it to make a judgment call. The Ranger games have the capability of sending Pokémon to some core series games, but this is not a bi-directional transfer. Pokémon Box and My Pokémon Ranch are technically bi-directionally compatible with some core series games, but as they are basically extended storage, I doubt I'll ever have much to say about them. Let's Go Pikachu and Eevee, despite sharing more mechanics with Pokémon Go, ultimately are considered core series games in my view based on their status as remakes of existing core series games. These are arbitrary definitions and you may disagree, but I will be consistent in my placement of content based on my personal interpretation.