What does the Type IAA / IA / II rating actually mean?

It's the duty rating set by ANSI. Type IAA is rated to 375 lb, Type IA to 300 lb, Type I to 250 lb, Type II to 225 lb. The rating covers the user plus tools and materials, so a 200 lb user carrying a 25 lb toolbelt and a gallon of paint is already pushing a Type II. For most homeowners doing real work, Type IA or IAA is the safer floor.