In the words of Electro-Harmonix:

The EHX Attack Decay can produce a single volume envelope like the original, or, with Poly mode engaged, put separate volume envelopes on each note you play. The built-in Harmonix fuzz is now a fully controllable distortion with gain, tone and volume controls. The effects loop allows the musician or sound designer to place the volume envelope onto whatever signal is present at the Return jack. Expression pedal/CV input and three user presets give the user total control over their Attack Decay!