My guess is that you could usefully play for a while with our Routing Utility (see H8000 support page). This would demonstrate the way to feed signals through the unit.
In practice, it usually doesn't matter which inputs and outputs you use, as long as they are correctly routed to the dsps. Each preset has different connection needs and will act differently if it is missing something. For example, an 8 channel equalizer won't care if you only give it four inputs. It will only give you four outputs, but they'll be fine. Whereas, a stereo reverb probably will lose something if you only give it a single channel.
It partly comes down to knowing the unit and the presets, but if something gives you a sound you like, that's a good thing, even if it could possibly give you more.
My recommendation is to study the Routing, and also the Presets Manual – note the difference between (say) stereo and dual mono.
