Picture shows only the master side for a headset:
Input on master side is left/right over AD0 and is being transmitted to the slave as front/primary right and front/primary left
Output on master side is left over AD1 and is being transmitted from the slave as microphone 0
These are design choices you can make in the configurator and can be set up in multiple ways. There is nothing preventing input being split over two ADx pins or input/output swapping ADx pins. Download the configurator and play around with it, it will give you a better understanding on how things works (the help system is extensive).
The IC was designed in the event we would manage to squeeze in 6 channels per device and thus 3 ADx pins were required. Once the protocol was made we could not make a robust enough solution to support 6 channels and had to limit our offering to 4.
Kjetil