You can teach Pokémon moves they wouldn’t normally learn in exchange for Battle Points in Pokémon Ultra Sun and Ultra Moon

In Pokémon Ultra Sun and Pokémon Ultra Moon, players will be able to teach their Pokémon moves in exchange for Battle Points (BP)—moves that they would not be able to learn by leveling up or through any Technical Machine ^TM. Players can earn BP by winning battles in the Alola region’s battle facilities, such as the Battle Royal Dome and the Battle Tree, or by using Mantine Surf from various beaches. Players can teach moves to the Pokémon that travel with them on their journey or even to the Legendary Pokémon that they catch passing through the Ultra Wormhole.