Rule 5 draft - Wikipedia
From the Link:
The Rule 5 draft is a Major League Baseball (MLB) player draft that occurs each year in December, at the annual Winter Meeting of general managers. The Rule 5 draft aims to prevent teams from stockpiling too many young players on their minor league affiliate teams when other organizations would be willing to have them play in the major leagues.[1]
I came upon this rule when I was searching for Kelly Gruber - who played for the Blue Jays. During an exhibition game when Toronto played the Syracuse Chiefs, (their AAA farm Team). I had the opportunity to go on the field during warm-up. I asked for and received his autograph. When in Syracuse, Kelly would come to our radio station, for interviews, as he was a believer.