<P> The current minor league classification system divides leagues into one of six classes, those being Triple - A (AAA), Double - A (AA), Class A-Advanced (High A or A+), Class A (Low A), Class A Short Season, and Rookie . Furthermore, Rookie is further informally subdivided into Rookie Advanced, complex - based Rookie and international summer baseball . Under the rules governing the affiliated minor leagues (specifically Major League Baseball Rule 51), Class A-Advanced, Class A, and Class A Short Season are separate classifications despite the similarity in name . </P> <P> Most leagues at Double - A and below play a split season, where the standings are reset at the halfway mark, and the winners of both halves qualify for post-season play . This allows teams to remain in contention longer into the season, as rosters feature heavy turnover during the course of the season . </P> <P> This classification currently includes two affiliated leagues: the International League and the Pacific Coast League, which feature teams from Eastern and Western United States respectively . For most of the 20th century, it also contained the American Association, based in the Midwest, but that league disbanded, its clubs absorbed by the other two leagues, as part of a reorganization of the Triple - A level in 1997 . The Mexican League is also classified as a Triple - A league, though its clubs do not have PDCs with Major League clubs . </P> <P> Both young players and veterans play for Triple - A teams . Teams usually hold many of the remaining 15 players of the 40 - man major league roster whom the major league club has chosen not to play at the major league level . Players at Triple - A on the 40 - man roster can be invited to come up to the major league club once the major league roster expands on September 1, although teams usually wait until their affiliates' playoff runs are over, should they qualify . For teams in contention for a pennant, it gives them fresh players . For those not in contention, it gives them an opportunity to evaluate their second - tier players against major - league competition . Some Triple - A players are "career minor leaguers", former prospects whose skill growth has halted and are not likely to advance to MLB, unless as a temporary replacement . </P>

When does the minor league baseball season end