Let' start with Channel. In this mode, voices 1-8 are received from MIDI Channels 1-8 accordingly. This mode is useful if you want to connect external sequencers like Seq or your favorite DAW. Using the jumpers on Poly back panel you can switch MIDI Channels to notes.
First and Next become handy when you want to play your modular using external controller. What is cool in these modes is that we implemented automatic gate recognition. So say you just want to control only two voices - you input whichever two gates.
In First mode when you press a key on external device first non-occupied voice from the top will be triggered. The use case here might be for example playing bass synth with your first note and then melody using different synth while holding the bass note.
Next mode works like voices do in most polyphonic synthesizers. When you send MIDI note, it will look for next non-occupied inserted gate and trigger it. Here you can get the very cool effect if you send voices to different-sounding modules. Then playing melody will scroll across those voices. Again, these are only basic examples.