The best way to figure it out is to hook up a simple modular patch with a Pad connected to the Envelope and Oscillator to the output. Then you can hear what it's doing.
I think the "PreDecay Time" and "PreDecay Level" would be like the Hold stage on some AHDSR envelopes. So after the envelope's Attack and before it's decay you can specify a length of time for the sound to hold there with the "PreDecay Time" knob and the level of the hold with the "PreDecay Level" knob. (I think there's a bug with the "PreDecay Level" when it's set at 1.00 though, so try it at .99).
Set a larger Release Time and try the "Release Slope" to hear how it controls the shape of the release. At one extreme the slope becomes convex, in the middle it's linear, and at the other extreme the slope is concave.