JSON mapgen questions

I was wondering if anyone could help with a couple of questions I have about the mapgen JSON? I’ve poked around in the wiki and the forums, but haven’t been able to locate the information myself.
First, what are the “chances” in the place_vehicles/place_items? Are they percentages? One in X? Something else?
Second, for the place_items, how does the “repeat” work and what do the numbers associated with it mean?