Yamaha DTXpress/DTXplorer/DTXtreme

I know you can adjust the volume, MVEL, and the pad output knob on 
the back of the pad, but what are the differences between all of 
them?  Can't you adjust just one of them and get the same effect?  I 
think some of those options though also have to do with how hard 
your hitting the pad and your playing style right?  Thanks!