The 9x19mm Parabellum round was originally created in 1902 for the Luger semi-automatic pistol. The caliber offered many advantages to other calibers, such as the .45 ACP round, such as it's low cost and the ability to easily offer double stack magazines. Soon, the 9mm round found its way to many Submachine Guns and Pistols, such as the UMP9 and the P92.
The 9mm is one of the most powerful forms of ammunition in the game, providing a high damage rate and fast launch.
It is officially the most commonly found ammo in the game, and can be located anywhere from the smallest barn to Sonsnovka Military Base.