The Communications Zone referred to an area of operations to the rear of the Combat Zone.
Communication Zone units are sorta like Divisional or Corps admin, communications and logistic units.
An example was the 2nd Signal Regiment, which was previously (40's & 50's) known as the 3rd Line of Communications Signal Regiment. It's Unit tac sign was red 2 / 303 on a white / blue background. The 303 was also used by other CommZ units.
In more modern times, the equivelant units would reside in 17 CSS Brigade and use its tac sign.