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. When a player is optioned to the Minors for a span of more than 20 days, he loses an option. Players who are optioned to the Minors are removed from a team's active 25-man roster but remain on the 40-man roster.
Upon being optioned to the Minor Leagues, a player must remain there for a minimum of 10 days before he is eligible to be recalled to the Major League roster, unless he is serving as the 26th man for a doubleheader or replacing a player who has been placed on the injured list. In these exceptions, there are no minimum number of days in which the optioned player must remain in the Minors.
For example, the Mariners optioned right-hander Dan Altavilla to Triple-A Tacoma on May 11, 2017. Altavilla would normally have been ineligible to return to the Major League roster until May 21, but he was recalled three days later, on May 14, when the Mariners placed right-hander Ryan Weber on the 10-day injured list.
An option applies to an entire season, meaning that a player can be sent to the Minors and recalled to the Majors any number of times over the course of a season while only losing one option. If he is again optioned to the Minors for 20 or more days in the following season, he would lose his second option and can once again be optioned to the Minors and recalled any number of times that year. The same is true of a player's third option year.
If a player misses an entire option year due to injury or expends his third option year before he has completed five professional seasons (Major Leagues and Minor Leagues included), he can receive a fourth option year.
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.
Impending rule change that will go into effect at the start of the 2020 season
Beginning in 2020, the minimum number of days a player will need to remain in the Minors after being optioned will increase from 10 to 15, with the same exceptions in play.
Mets shortstop Wilmer Flores entered the 2015 season out of options, having been added to the 40-man roster in November 2011. Flores was optioned to the Minor Leagues for the entire 2012 season and spent significant chunks of the 2013 and 2014 seasons in the Minors as well.
Then-Padres outfielder Rymer Liriano missed the entire 2013 season -- an option year -- due to Tommy John surgery and was rewarded with a fourth option year as a result.