From the manual:
Each Matrix layer provides real-time metering, level, pan, mute and solo control over the 4 stereo paths on the Home > Edit screen, 8 stereo tracks on the Song screen, Click, Count In, USB In 1/2 (for monitoring your DAW), Bluetooth audio (for jamming along with Apple Music®, Spotify®, or YouTube™), and Aux In.
A matrix layer refers to the physical Outputs, 1/4”, XLR, and Phones. So the quote suggests you can have different Song mixes for each output but you can’t have different mixes for different Songs using the same output.