Each winter, Major League Baseball’s Rule 5 Draft provides clubs the opportunity to pluck players from opposing franchises, provided they meet certain qualifications. Players who were signed at age 18 or younger must be put on a team’s 40-man roster within five years or risk being exposed to said draft, while players signed aged 19 and over must be added within four years to be ‘protected.’
Effectively, this is a built-in mechanism to keep franchises from stockpiling players in their minor leagues who would otherwise merit advancement, as well as giving the players involved a more robust chance to climb the professional ranks even if they were previously in a loaded minor league system.