Pokémon Legends: Arceus is the first Pokémon game set long before the modern era. Every other Pokémon RPG to date has relied on PCs and digitized boxes to store Pokémon collected throughout the game. However, despite the existence of a magical smartphone, there are no modern computers in Pokémon Legends: Arceus, which means that the game's storage mechanics will be different from its predecessors. The results appear to be closer to the mechanics of early pre-smartphone Pokémon games than more recent generations, such as Sword and Shield.
Pokémon Legends: Arceus introduces pastures as a method of storing Pokémon. Players will be able to take up to six Pokémon with them on an investigation mission, which will be stored in Poké Balls on the player's person. All other Pokémon will remain in the pastures, which means that players cannot switch Pokémon out of their team mid-mission.
The official site specifically uses the word "pastures" in the plural. It is assumed that most of the cities in the game will have their own pasture and that the player's Pokémon will not magically materialize in a pasture other than the one they were left in. Spreading Pokémon into distinct pastures would bring a whole new level of planning to the franchise, as every PC in modern games contains all of the player's Pokémon. However, this conjecture has yet to be confirmed by The Pokémon Company.
Pokémon storage details are one of the core gameplay mechanics of any Pokémon RPG. Even the smallest formula changes can drastically change a game's difficulty. Pastures will reintroduce a scarcity to Pokémon holidays not seen in recent Pokémon games. In Sword and Shield, for example, the advent of smartphones meant that PCs previously only available in Pokémon Centers could now be in the player's pocket, accessible at any time. In turn, this meant that the scarcity of the party was completely eliminated from the game: if a Pokémon from the party was passed out, the player could immediately replace it with a healthy Pokémon in its box, even halfway through. Pastures will likely be a welcome return to a more challenging style of play.
