Hi Frank,
My first post, so want to say Hi to all. I'm a Load Controller in Aus, so I hope my own questions may be accepted within the forum.
Your question intrigued me, particularly as I was not licensed on the 767's. However, the basics are the same for all aircraft types. Your answer has come in part from what I have read.
The ZFW is the structural limit of the aircraft, before you look at ramp fuel, taxi-out burn, BRW and LW. I feel that the term "cargo" used by Boeing refers to a broad term depending on intended usage.
Hey, you're in a Sim, just place the 19 tonne under the seat in front of you, or in the overhead locker.
Regards, Phil