The unique Dual Band mode of the nvelope takes you to the next level of impulse shaping. The attack and sustain parameters become much more flexible by selectively processing a certain frequency range chosen by the user.
The Freq A controller now sets the start frequency for attack processing, while the Freq S controller determines the end frequency for sustain processing. The function of the Attack and Sustain controllers remains the same as in Full Range mode.
So it is pretty likely you'll find yourself using the Dual Band mode in any situation where you would like to adapt the way the nvelope processes more individually and on material which tends to produce unwanted artifacts in Full Range mode.
Generally speaking, Dual Band mode gives you more distinct processing options and the possibility to even cope with complex material like a summing bus.