The AD-10's STEREO/MONO switch sets the XLR OUT L/R jacks to stereo or mono (dual mono).
In STEREO, route XLR OUT L and R to two separate mixer channels, then hard pan them left and right for true stereo monitoring. The difference is most noticeable with panned sounds or panning effects.
In MONO, XLR OUT L and R output the same summed signal (dual mono).
This switch applies to XLR OUT L/R only. It does not affect LINE OUT L/PHONES or R/MONO.