AX stands for 'Arenacross", which consists essentially of Supercross-style courses that are downsized even further and placed into smaller venues such as ice hockey and basketball stadiums. The popularity of Arenacross is growing however, since smaller cities that do not have a large football or baseball stadium can host Arenacross races.
The AMA is using AMA Amsoil Arenacross series as a way for riders to transition from amateur racing to Supercross. Many Supercross privateers race Arenacross. Some of the top racers of Arenacross include: