Players on the 25-man roster which is also called the Major League Roster can be sent down to the Minor Leagues at any given time as long as they have options remaining on their clock. A player has 3 options assigned to them when they're added to their team's 40-man roster. An option is good for the entire year so they can use 1 option as much as they want for that season but the next season it goes to option 2 if he's sent back down to the Minor Leagues. If a team tries to send a player down to the Minor Leagues but he has no more options left then he has to clear waivers before he can be sent down. There's also the possibility of a player having a 4th option player but this only applies if a player has less than 5 years of professional experience with their particular team whether it's in the Minor Leagues or in the Major Leagues or a combination of both.
Also when a team sends a player down to the Minor League, a player has the right to refuse that assignment if they have 5 years of Major League Service however refusing such an assignment means he typically will automatically become a free agent in most cases.
Also when a player is sent down, they have to stay down for a maximum of at least 10 however if an injury occurs to a player on the Major League Roster and the player is placed on the DL then the player that was just sent down can be brought back up regardless of the 10-day limit thing.
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The only rule involving calling a player up from the Minor Leagues to the Major Leagues has to do with limits providing they've been recently called up then sent back them then they need to stay down in the Minors for a minimum of 10 days unless it's in the event of an injury situation that results in the injured player going to the DL then the player that was recently sent back down can be brought back up regardless of how long he's been back in the Minor Leagues.
They are an amuature league that teams can send players down to to improve them.