Well, I have an AR7700 installed on my FPV wing and I have a DX8. I have full use of 8 channels with this receiver. the way that I understand it is this, whether you use SXRL or PPM, each channel sends a signal, one after the other, through the same protocol. So, instead of using multiple wires, from an AR8000 for example, you are just using one servo wire to send multiple signals in sequence.
does this help?
just an addition...I have my AR7700 connected to a flight controller on my FPV wing and I'm using Inav (a branch off from Cleanflight and Betaflight). After setting up the receiver settings in the GUI, I was pleased to see each channel working in the Receiver Tab of the configurator.