Volume (midi code 7) and Expression (midi code 11) COULD be used interchangeably since they do the same thing, however, volume is usually a 'fixed' thing while expression is usually used for crescendos and decrescendos. As I'm sure you've noticed, they are labeled separately in music stores and also are usually different in appearance with expression pedals usually larger and with longer physical throws. This would make it more nuanced (and dynamic) through the range.

chas