90% refer to what?

I am wondering what the 90% in option 2 refer to. If I collect and use rainwater, I may use 90% of the annual rainwater on the site. But if I have any other of the listed sources (condensate from AHU cooling coils will be available almost everywhere), can I get the 90% point without using any other source than rainwater? For the example rainwater, how would that be calculated? Site area x annual precipitation x run-off coefficient? I think this is not really clear. Additionally, how would "whether it meets the local standard wastewater" be documented?