In appendix, Para 1.2 Definitions and specifications, the definition for "Payload" states

Payload should be the Volume Limited Payload (...) or the Maximum Zero Fuel Weight limited payload, whichever is the lesser.

How can we can define and understand "Maximum Zero Fuel Weight limited payload"?

• In short, the Max Zero Fuel Weight payload will be that which brings the total aircraft weight up to that limit. The Max Zero Fuel Weight limit is a structural limit to prevent damage to the wing spars. No time for a more thorough answer at this time but I'm sure someone will pipe in soon! – Porcupine911 Feb 15 '16 at 16:55
• Though I have not heard the term Volume Limited Payload before, I suspect they're talking about a passenger airplane. In pax aircraft you typically run out of empty seats before you bump up against the MZFW, In freighters you fill the air above each passenger with cargo and are often limited by the MZFW. – Terry Feb 28 '16 at 22:02

The Maximum Zero Fuel Weight Payload, is the amount of payload $m_{pl}$ (i.e.: passengers, cargo, but not fuel) required to reach the Maximum Zero Fuel Weight condition while keeping the fuel weight, $m_{fuel}$ to zeros so:
$$while \; m_{fuel} =0: \\ MZFW = OEW + m_{pl} \\$$
The Maximum Zero Fuel Weight ($MZFW$) is an indicator of the maximum loads that the wing structure can bear. For the wing an "unloaded" situation (no fuel in the wing) is critical, since the fuel stored in the wing helps reducing the loads on it in flight. The lift acts "upward" while fuel weight acts "downward", thus the fuel contribution helps relieving loads from the wing structure.
The volume limited payload is an indicator for the sizing of the cargo compartment structures. These are the structures of the cargo area, where the LD containers are stored (under the floor of the passenger cabin). This Volume Limited Payload is set to avoid having too "dense" LD containers, the limit of density for the containers is around 350 $kg/m^3$, you can find some technical specifications for LD container here.