The same Envelopes in the C4 can control two different things, which are the filter and the amplitude. There are only two envelopes, but they are performing multiple jobs in this case (or you can utilize both of them independently so one envelope is only controlling the amplitude of the voices and the other is controlling the filter if you only assign each envelope to separate duties). On a typical analog synth you will have two envelopes with seperate controls, one dedicated to the filter and one dedicated to the amplitude (often just called "Amp"). Its somewhat confusing with a bass triggering the synth because in many cases you are probably trying to use the envelope of your bass playing to control the amplitude of the synth, but you can instead get interesting sounds by using one of the internal C4 envelopes to subtly shape voices (you are somewhat limited here due to not having full ASDR controls for the envelope like you would on most synths) while just using the bass to trigger the voices/pitch.
The "Envelope" dropdown is selecting to use one of the two envelopes to control the amplitude of the voice (so for example you could have the voice swell up slowly in volume by choosing "Envelope 1" under the Voice's "Envelope" dropdown then choosing the "swell" envelope for the "Type" under that Envelope's parameter menu), while the filter envelope is selected under the Filter menu dropdowns and assigned to each voice by the Processor Pan knob (sending the voice to either Distortion/ Filter 1 and/or Distortion/ Filter 2) and the Destination menu (so choosing whether or not that voice gets sent to the distortion and/or filter of the processor you have panned to).
It can be pretty confusing until you wrap your head around the signal path and how it all ties together!
Hey thanks so much for your full answer here! Really appreciated.
I hadnt thought of it before as the 2 envelopes doing multiple things, separately or together (although Id probably already read it somewhere?!)
And of course, the pluck on the string has its own /envelope' characteristics!
My attempts at my own presets are somewhat hit & miss, but this helps .