Pre-arb means the layer does not have enough service time to enter arbitration (typically 3 years). The Brewers can renew Mitchell at the minimum pre-arby salary but many teams do have small incentives to raise salaries above the minimum especially for starters. You can sign a player to a contract the replaces their arbitration years, but that makes the contract guaranteed where under arbitration you can non-tender the player without any salary owed. For Mitchell, he's just hardly played so it's really hard to guess about what to do.