under the surface of the gym rework update
v0.67.1 APK Datamine
8. premier balls
Based on a series of factors, Premier Balls will be awarded at the end of a Raid Battle to help catch the "powerful Pokémon of your own." From what has been found, these awarded balls are the ONLY PokéBalls you'll be allowed to use to capture the powerful Pokémon, so don't slack off in Raid Battles. The factors are the following: Base of PokéBalls, contribution in the Raid, Team contribution in the Raid, Gym ownership, Team's damage percentage, and the mysterious "Bonus Item Multiplier."