Pokémon Legends: Arceus promises to make major changes to the proven Pokémon RPG formula. However, the premise of the Pokémon games hasn't been completely reinvented, and there are some proxy devices that have crept in to make Pokémon Legends: Arceus more recognizable. One such device is the Arc Phone, which shares a design aesthetic with the game's owner Mythical Pokémon.
According to the game's official website, players will receive the Arc Phone at the start of the game. The Arc Phone contains "some kind of weird power" and has a really mysterious connection with Arceus. Similar to Sheikah Slate from The Legend of Zelda: Breath of the Wild, the Arc Phone appears to function as an interactive map that also lists the player's current missions. Although not confirmed, the Arc Phone is likely to serve as the game's Pokédex as well.
Pokémon Legends: Arceus is set long before the modern era of major Pokémon games, in an environment similar to feudal Japan. Despite this, the Arc Phone appears to be a version of a smartphone powered by the Pokémon god. Interestingly, Breath of the Wild's Sheikah Slate is similarly powered by ancient technology.
However, there appear to be limitations to the Arc Phone compared to a modern smartphone or PC. In Sword and Shield, for example, players can swap their team's Pokémon for those in the boxes at any time. But before the advent of ultra-powerful handheld computers, the first several generations of Pokémon games allowed players to trade Pokémon only on PCs in Pokémon Centers. Pokémon Legends: Arceus is returning to this style of play. Players can only hold six Pokémon at a time, via Poké Balls on their person. The rest of their beloved friends will be held in the pastures, just like in the old days.