Battle Party

A Battle Party is a loadout system that allows a Trainer to create and name teams of up to six Pokémon in their Pokémon collection for quick access when participating in Gym battles and Raid Battles.

Background
Battle parties are preset teams of up to six Pokémon that Trainer are able to create and customize to fit their needs. Trainers can create different parties for frequent Raid Battles, or have battle parties that take advantage of certain weather bonuses.

During the initial beta phase, battle parties will be saved to a single device, so parties will not transfer if a Trainer buys a new device or uses multiple devices to play Pokémon GO. Once the feature is fully launched, battle parties will carry over to each device that a Trainer logs into.

With the new update and trainer battles returning, we now have parties with only 3 slots for trainer battles. Since there return, ditto has been banned.

Trivia

 * The Battle Party system was first discovered in the game's code with the release of the 0.67.0 APK.
 * The system was rolled out in version 0.85.1.