For most users, although the information in
https://wiki.jriver.com/index.php/Parametric_Equalizer#Mixing is clear enough to intuitively understand the mix channels function, users who are not familiar with the most of the terminology mentioned there may find them hard to understand. I am one of them and it took a while to know exactly what happens with each of the function.
Here below are the effect after checking the relevant boxes in the mix channels filter:-
Copy Left to Right means both Left and Right speakers will play the left channels. No Right Channel sound will come from the speakers.
Move Left to Right means Left Channel only will be played on the Right speaker. The Right Channel sound will not come from the Right speaker and No sound from the left speakers.
Swap Left to Right means the channels are reversed and you will hear right channel in the left speaker and left channel in the right speaker.
Add Left to Right means Left channel is added to Right speaker. Right and Left Channel sound will come out from the Right speaker. The left channel sound will come out of the Left speaker only.