The way Major League Baseball sets up their pay scale for young players, there are three years of team control where the player makes league minimum and has a minor league “option”. If that player spends more than 20 days in the minor leagues during the season, their option for that year will be used up. The Milwaukee Brewers have managed this carefully.
Most players have their minor league options used up by their arbitration years. Those who don’t get sent down to the minors at all during those first three years retain those options until free agency.