Inspecting PV systems

I know most of us disclaim them, by FYI here’s a good article on inspecting PV installations:

“…some systems will be installed with a dc grounding electrode conductor connected to a** dc grounding electrode** or to the** ac grounding electrode**.”

What’s the difference? And what’s the difference between an ac GEC and a dc GEC?

Since the code requires all ground electrodes on a premisis to be bonded together in one grounding system it is somewhat moot.