The Stahl Arms logo the private army wears.
The Private Army of Stahl Arms is a personal military organization that answers to the direct authority of Chairman Stahl, serving more as corporate mercenaries loyal to the one who signs their paychecks as opposed to the legitimate Helghast Army and it's loyalty to the state. They have been described and witnessed as a large and formidable force, performing a variety of different jobs. They act as security personnel for the Stahl Arms Deep South facility, combat units in the field while hunting ISA troops, and even perform more mundane work such as firearm testing and weapon construction in the factories. This hints that the private army outnumbers non-military workers in Stahl Arms, and may perhaps have already absorbed the entire civillian body in the company.
The assets of the private army extend beyond basic infantry units, containing a variety of other soldiers in their ranks. Some mirror troops in the regular Helghast Army, such as shock troopers, support troopers and riflemen. But there are a few particular units that are unique to the private army such as the Hazmat Troopers, Capture Troopers, and jetpack units. In terms of armored support the private army is lacking aside from troop carriers and overlord dropships, most of their efforts focused in the mass production of infantry related equipment.