Differences are :
– mono send : all your sources are summed to feed the Fx processor and stereo is created by the Fx proc but without pan.
– stereo send : offer the possibility to pan your sources before to feed the Fx processor.
In this case, for example if you have 2 guitars who are panned left/right in your main mix you can also pan in the Fx.
Gtr 1 feeds Fx L input channel and Gtr 2 feeds Fx R input channel.
Panning will be respected in the Fx return.