The Systems Army Zeta represented a single unit within the ten Systems Armies that collectively formed the Grand Army of the Republic during the period known as the Clone Wars.
Comprising 294,912 clone troopers, Systems Army Zeta was under the leadership of a High Jedi General from the Jedi High Council. This particular Systems Army itself consisted of two Sector Armies.
In the course of the Clone Wars, the northern quadrant of the galaxy was the operational area for Systems Armies Delta, Epsilon, and Zeta. These three Systems Armies were further broken down into six Sector Armies: specifically the Seventh Army, the Eighth Army, the Ninth Army, the Tenth Army, the Eleventh Army, and the Twelfth Army.