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Normally, a player on an MLB Reserve List (40-man Roster) is allotted three minor league option years. This means the player can be sent to the minors and be available 24/7 for recall back to the major league club whenever the player might be needed during the course of three different seasons.
1. If a player spends 20 or more days of an MLB regular season on Optional Assignment to the minors, an option year is expended and the player does not accrue MLB Service Time while on Optional Assignment. If a player spends 19 or fewer days of an MLB regular season on Optional Assignment, an option year is NOT expended and the player does accrue MLB Service Time while on Optional Assignment.
2. After three minor league option years have been spent, the player is said to be "out of options" and the only way the club can send the player to the minors is by removing the player from the 40-man roster and sending him "outright" to the minors. And the club can do this only after first securing Outright Assignment Waivers (which are irrevocable) from the other 29 MLB clubs.