About Ref. input and Ref. output

Hi everyone!
I'm measuring Ambisonic plugins via REW. I'm using the program's measurement system, sending the signal to Reaper via a virtual cable, and it returns via a virtual cable.

Currently, I'm using two channels, but I don't understand the reference input and reference output channels. I have to specify them, but there's no signal other than what the program sends to Reaper.

What would be the optimal configuration in this case?

Thanks for the help!

If you aren't using a timing reference it doesn't matter what they are set to.

And what if I want to use a loopback as timing reference?

You need a signal path that bypasses the plugin if you want to measure delays from the plugin.