What does the number mean in isuzu trucks ? ie NPR 200, NPR 300, NPR 400 ?

[aussie961]

ON the side of isuzu trucks they have the letters to show the model and a number..

ie nlr 200, npr 200, npr 300.

I know what the letters mean but what do the numbers mean ? I know the bigger the truck, the bigger the number but what does it measure ?