Well, simply reading the manual, it seems that you just select the input source you want, and then you can cycle through all the digital inputs to select the one you want for that source. Kind of an odd way to do digital audio input assignment, but whatever works.
So looks like you can use any of the six digital audio inputs with any of the input source selections.
So looks like you can use any of the six digital audio inputs with any of the input source selections.