The Gym rework update has begun distribution, so lets begin looking over everything in the APK and GAME_MASTER file, beginning with Premier Balls.

Premier Balls have been added to the game's code, and will be the first new PokéBall added into Pokémon GO. The only difference between them and a regular PokéBalls is the design. These specially-designed balls will be awarded at the end of Raid Battles to capture the Raid Pokémon. These special balls with be the only balls used to capture the Raid Pokémon. Once your supply of Premier Balls is depleted the Raid Pokémon will run away. Before you panic, your regular supply of Berries will be available to you when capturing. Golden Razz Berries will be essential when using Premier Balls. At the end of the capture, any leftover Premier Balls will disappear from your inventory.

Premier Balls are a part of a Raid Battle reward. Many factors relating to your participation in the Raid determine the amount of Premier Balls you'll receive. Make sure you throw accurately and use your Berries!

GO Hub is the source of this information