It was developed in the mid to late 1970's for use by vehicle crews, but is also popular with law enforcement units and special forces. The U stands for "Ukorochenniy", which is Russian for "shortened". Especially in the USA the AKS-74U is better known as the Krinkov.
The RUS-79U submachine gun in Call of Duty Mobile is a prized acquisition that can be unlocked. The weapon is known for its exceptional rate of fire and manageable recoil, making it a popular choice for players who prefer a rapid playstyle, particularly in close-quarter combat scenarios.