From BR Bullpen

While the minor league classification system is in theory rather clear, the A designation currently covers two kinds of leagues, and until the 2021 season, that figure was three: A-Advanced, also known as High-A; A, also known as Low-A; and A-Short Season, also known as Short-A. As a resulthere was a huge leap in talent from the bottom to the top end of what was in theory a single level.

When Major League Baseball took over and downsized Minor League Baseball after the last Professional Baseball Agreement expired in September 2020, Short-A and the next level down, Rookie-Advanced, were eliminated . This therefore leaves only High-A and Low-A as operative classifications at the Class A level at this time.