Players become free agents upon reaching six years of Major League service time or when they are released from their organization prior to reaching six years of service time. A free agent is eligible to sign with any club for any terms to which the two parties can agree. If a player with fewer than six years of service time signs with a club, he remains under the control of that club until reaching the requisite service time to reach free agency -- even if the contract he signed does not cover the remaining years until that point.
Jason Heyward made his Major League debut on Opening Day in 2010 and never returned to the Minor Leagues. He reached six years of Major League service time at the completion of the 2015 season, at which point he became eligible for free agency. Heyward went on to sign an eight-year contract with the Cubs.
Tony Sipp was released by the Padres in May 2014 when he had between four and five years of Major League service time. The left-hander subsequently signed a guaranteed Major League contract with the Astros that ran through the end of the '14 campaign. Because he finished the season with less than six years of Major League service time, Sipp was eligible for salary arbitration as opposed to free agency. Sipp qualified as a free agent following the 2015 season and re-signed with the Astros on a three-year deal.