Five to 6 inches of reinforced concrete carries passenger cars. Truck routes, loading areas, and dumpster pads need 6 to 8 inches. The expensive mistake is pouring the whole lot at car thickness and letting the delivery trucks find the thin spots, so the scope prices thickness by traffic area.
More detail
Parking design starts with an honest traffic map. Passenger stalls and drive aisles that will only ever see cars and pickups perform well at 5 to 6 inches of reinforced concrete over a compacted base. The areas that need more are knowable in advance: the route a delivery truck takes to the back door, the loading zone, the fire lane the city requires (and which must carry a loaded fire apparatus by definition), and above all the dumpster pad and its approach, where a fully loaded garbage truck applies its axle load twice a week in exactly the same spot for the life of the lot. Those areas run 6 to 8 inches, and pricing them separately at pour time is far cheaper than the alternative, which is watching the thin version spall and break in the first few years and repouring it with the business open. The companion specs carry their usual weight: reinforcement sized to the thickness, joints laid out so the big rectangles crack where they are told to, subgrade compacted and moisture-conditioned like every other slab on this clay, and drainage designed so the lot sheds to inlets or grade instead of ponding against the building. City plan review shapes some of it (stall counts, accessible stalls and routes, fire lanes, sometimes landscape islands), and the written scope maps thickness by area against that plan so the lot is priced for the traffic each square foot will actually see.