The envelope signal is a measure of the instantaneous level of your signal where as the ADSR has 4 discrete states (attack, decay, sustain, and release). In other words, the envelope just follows the level of your playing and is therefore much more sensitive to dynamics while the ADSR is triggered by an attack then goes though all 4 states so it's much more predictable and stable.
That would make sense if there were controls for attack, decay, sustain and release, but the Modfactor doesn't seem to have them. Are they just preset at the factory? If so, what is the TC for A, D and R and the level for S.