Battle Party

A Battle Party is a loadout system that allows a Trainer to create and name teams of up to six (three in Trainer Battle) 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 was saved to a single device, so parties did not transfer if a Trainer bought a new device or used multiple devices to play Pokémon GO. Once the feature was fully launched, battle parties carried over to each device that a Trainer logs into.

With the introduction of Trainer Battle, Trainer can construct three battle parties for each league. However, only three Pokémon can be assigned in the battle party of this mode.

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.