My opinion only, I don’t have a link.
The term assault rifle has two meanings. First is the legal one, which keeps changing, and is essentially meaningless. Pretty soon BB guns will be assault rifles.
The military definition is a different. By it, an assault rifle is a rifle that is optimized for use by a foot soldier to lay down suppressive fire while running towards, or “assaulting”, a position held by enemy troops. It has the following characteristics:
It is portable enough that the foot soldier can carry it and a large quantity of ammunition for it while moving.
It is full auto, so a lot of lead can be sent downrange quickly.
It uses high-capacity, easily changed magazines, for the same reason.
The round it uses has more power than the pistol cartridge of a sub-machine gun but is less powerful than a battle rifle round. The 7.62X39 is the perfect example of this intermediate round. This intermediate power round allows reasonably accurate full auto firing while moving. The substantial recoil of the tradition battle rifle hinders its use in this role use by most soldiers.
On the other hand, the assault rifle lacks the long range and penetration capabilities of the battle rifle.