Chassis is a reference to the bottom or base of a diecast car. In most cases the chassis has the frame, exhaust, and driveline as well as the car name molded or cast into it. It also includes the Country where it was made and the year of copyright.
However, some cars like the Peterbilt Dump Truck (from the Auto City gift pack) had no car name. It only included the Country of Origin (Creation) and the year.