Runoff %.. what does that mean exactly?

Stupid question, I know, but what exactly does run off percentage mean? For example, someone said to flush organic soil to 100% runoff with clearex.. what does that mean? thanks !

100% run off would mean if you dumped in a gallon of water, you'd want gallon of water to run out the bottom.. idk how you'd ever get 100% run off though tbh as the medium is almost always going to absorb at least some of the water going in making it near impossible to get 100% imo..