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.
For manual triggering there is a button for each envelope.
CV control: Both attack and decay time can be CV controlled; the CV "multiplies" the time by 1...approx. 8 times.
More information and examples on the AE Modular Wiki.
(Price without VAT; for buyers inside the EU 19% VAT will be added to the shown price = 53,55€)