It’s not? What is it then?
All chickens are birds, but not all birds are chickens. All carbines are rifles, but not all rifles are carbines. Likewise, all assault rifles are rifles, but not all rifles are assault rifles. Some carbines are assault rifles. All three categories describe specific, functional features.
Rifle - a long arm with a rifled barrel.
Carbine - a rifle with a shorter barrel.
Assault rifle - a rifle (or carbine) capable of select fire and chambered for an intermediate cartridge.