In Alola, when breeding Pokémon whose offspring has a regional variant, the offspring's form depends on its parents. If the female parent is the species' normal form (including in a Ditto pairing) and holds an Everstone, and if the offspring is in the same family as that parent, then the offspring will hatch as the normal form. In all other cases, the offspring will always hatch in its Alola Form.
In Pokémon Sun and Moon, if a Pokémon evolves from a species that does not have an Alola Form to one that does, such as Cubone, it will always evolve into its Alola Form and cannot evolve into its normal form. In Pokémon Ultra Sun and Ultra Moon, the Pokémon will evolve into its Alola Form while in Alola itself, but can also evolve into its normal form while in Ultra Space.
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So a Kantonian form isn't transfer-only if you're playing USM and the Pokemon evolves from a Pokemon with no Alolan form (such as how Exeggcute can evolve into either Exeggutor).
