You can put the envelope at any level. So it's a choice about whether you want to use the envelope across multiple keys or layers, or it's just specific to one layer or key. However, no matter where the envelope is, it can only modulate parameters (gain, pan, Freq etc) that are at the keygroup level. Reason being, a keygroup is a "voice" is traditional synth language. Each time you press a key on the keyboard, a unique instance of a voice is started, and it will have a unique instance of every envelope that modulates it's parameters. So if you press and hold one key, all it's envelopes will trigger, then whilst keeping the first key pressed, you press and hold another key, it will also get all its own envelopes,vthe original keys sound isn't affected, it doesn't retrigger any envelopes on the first key. Old real synths sometimes had global envelopes and frankly it sounds terrible, hence why I presume uvi deliberately forbade enveloping of parameters above keygroup level, as it would affect all in-flight voices, not just the newest voice.