What is the definition of "the administrative area of the transit city"?

I have a flight coming into and leaving Beijing (PEK) with a 14 hour layover. It says I can't leave the "administrative area of the transit city" with the 72 hour free option. What does that mean? Would I be able to visit the Great Wall? I see that there are companies that offer tours from the airport to the Great Wall of China. Does that mean it's okay, or are they breaking the 72 hour visa-free rules?