Players on a 40-man roster are given three Minor League "options." An option allows that player to be sent to the Minor Leagues ("optioned") without first being subjected to waivers. Players who are optioned to the Minors are removed from a team's active 26-man roster but remain on the 40-man roster.
A player who is on the 40-man roster but does not open the season on the 26-man roster or the injured list must be optioned to the Minor Leagues. Once an optioned player has spent at least 20 days in the Minors in a given season, he loses one of his options. Only one Minor League option is used per season, regardless of how many times a player is optioned to and from the Minors over the course of a given season. Out-of-options players must be designated for assignment -- which removes them from the 40-man roster -- and passed through outright waivers before being eligible to be sent to the Minors.
Players typically have three option years, but those who have accrued less than five full seasons (including both the Major and Minors) are eligible for a fourth if their three options have been exhausted already. For the purposes of this rule, spending at least 90 days on an active Major League or Minor League roster during a given season counts as one full season. Players also earn a full season if they spend at least 30 days on an active Major League or Minor League roster AND their active-roster and injured-list time amounts to at least 90 days in a given season.
Upon being optioned to the Minor Leagues, a position player must remain there for a minimum of 10 days before he is eligible to be recalled to the Major League roster. For pitchers, the minimum is 15 days. If a player is serving as the 27th man for a doubleheader or replacing a player who has been placed on the injured list, there is no minimum number of days for which the optioned player must remain in the Minors.
A player's option years do not need to be used in succession. Any player with fewer than five years of Major League service time and an option year remaining can be optioned to the Minor Leagues. Players with more than five years of service time must consent to being optioned.