Payload
Payload is the permitted mass added to a vehicle by occupants, luggage, accessories, and other carried loads. Depending on the applicable rules and vehicle instructions, the vertical load imposed by a trailer on the towball may also use part of the available payload.
Payload is a shared limit
Payload is not a separate compartment rating. Every passenger and carried item consumes the same mass allowance, including equipment fitted after the published curb-weight configuration. A vehicle can run out of payload before its seats or cargo area are physically full.
A simple estimate is maximum rated vehicle mass minus curb weight, but the two inputs must refer to compatible definitions and the same variant. Market equipment, wheel packages, seating layouts, and factory options can change the result. The payload printed on a vehicle-specific label or stated in its official documentation is therefore more reliable than a generic model-page figure.
Payload is also different from towing capacity: a trailer has its own mass limit, while its towball download can reduce what may be carried in the vehicle. See Vehicle Dimensions and Mass for the connected mass definitions.