Outside of gun industry licensing for manufacturing and dealing firearms, few people discuss “classes” of weapons. Generally speaking, you have guns that are regulated by the National Firearms Act (machine guns, short barrel rifles and shotguns, silencers, destructive devices and similar things) and guns that are not. NFA items need a special Class 03 license to sell. However, the ATF classifies any rifle made in the US that isn’t an NFA item as Class II.
Unless made into a short barrel rifles, AR-15’s are not treated any differently by the ATF than any other modern firearm. There is no real need to discuss the “class” of an AR-15 as it’s either an NFA item or it isn’t. Some states sadly have restrictive laws that use technologically inaccurate terminology like “assault weapon” to classify guns based on cosmetic and other simple design features.