The 40-man roster includes a combination of players on the 26-man roster (it was 25, prior to 2020), the 7-, 10- and 15-day injured lists, the bereavement/family medical emergency list and the paternity leave list, as well as some Minor Leaguers.
In order for a club to add a player to the 26-man roster, the player must be on the 40-man roster. If a club with a full 40-man roster wishes to promote a Minor League player that is not on the 40-man roster, it must first remove a player from the 40-man roster -- either by designating a player's contract for assignment, trading a player, releasing a player or transferring a player to the 60-day injured list.
A player who is on the 40-man roster but does not open the season on the 26-man roster must be optioned to the Minor Leagues. Once a player has spent at least 20 days in the Minors on an optional assignment, one of his options is expended for that season. 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. Players typically have three option years, although a fourth may be granted in certain cases. 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.
The 40-man roster is an important distinction in the offseason, as players who are on the 40-man roster are protected from being selected by another organization in the annual Rule 5 Draft, held each year in December at the Winter Meetings. Players signed at age 18 or younger need to be added to their club's 40-Man roster within five seasons or they become eligible for the Rule 5 Draft. Players who signed at age 19 or older need to be protected within four seasons.