Rookies are typically high school signees, while Class A short season players have often attended college. The main objects during the first year are to teach or reinforce basic skills and just to get the player accustomed to playing daily and professionally. A second summer in short season is also common. There are lots of exceptions to this pattern. The dynamics differ a bit from system to system.
Some organizations have five farms in Class A and Rookie, while others get by with three.
Players get promoted because they've earned it or because someone got hurt or because the farm director thinks they might learn more from a different coach. Players get sent down, of course. Pitchers are more volatile , because their learning curves are different and they have different injury patterns. But the pattern I've described is common, and the MWL's set role in baseball's development system reflects these assumptions. Each MWL team is an affiliate of a major league team, and the relationship between the teams is defined by a player development contract.