Iīm not able to explain it from a technical Point of View, rather from a practical one:
Sustain in the Pro2 is rly just the Level/Volume the note settles at after the decay phase. It is not the duration!
Duration is controlled by you keeping yr finger down or the number of steps a note is on hold for in the sequencer (tie function). At full sustain If u modulate amp env release with lets say a random lfo u will get random note lengths at full lvl, but only after letting go of the key. At 0 sustain if u modulate amp env decay with a random lfo itīll result in random note lengths which also lower in volume over time. Modulating sustain would only result in changing the volume of your played notes. For that you can use VCA or Amp Env Amount as mod destinations. Modulating VCA will result in infinite sustain no matter what yr release is sat at, Env Amount changes volume for the duration of the Envelope. Same principle applies to the Filter and the auxilary envelopes.
I confused myself quite a bit writing this, maybe somebody else can elaborate.
The Pro2 rly is my first actual Synth, so i donīt know how it works with other machines.