ENV is a dual attack - decay envelope; it can be switched between full sustain (the output remains at full level as long as the GATE input is on) and zero sustain (for percussive envelopes). Another switch (SLOW/FAST) allows selecting the time range.
Ive been doing a bit of "research" on the AE, with an oscilloscope - and noticed that the 'old' VCA is not attenuating audio signals as you'd expect, its attenuating centre on 0v, not 2.5v as expected... and I'm wondering if the new DC/Audio switch does this. as this is precisely what the ac/audio switch does on the 4att/mix (and im guessing the 4io?) robertlanger?
Yes, did get the 2 VCA as well, all I can say is it does its job well, but I don't do any voltage checking or anything. I've only used the new ones on audio so far; moved the old ones to be modulation levels in my layout. The 3rd row at the top just has the Wavefolder and S/H modules at present (with racklink)