Q: What is the difference is between MIDI volume and expression?
A: Volume (Continuous Controller 07) and expression (Continuous Controller 11) both do the exact same thing. However they give users two different ways to deal with MIDI mixing. The standard protocol is to use MIDI volume (CC #07) to set the MIDI channel (or a phrase or segment in a sequencer) level for balancing overall volumes against other channels. A mixing console will accomplish the same thing but using CC #07 will do it in MIDI and thus lock it into the sequencer.
Expression (CC #11) can then be used for crescendos and decrescendos during the performance of the music. This way, if you need to bring the overall volume of the whole track up or down, you would use CC #07, but all the expression programming would still work just as you’d set it up to.