The envelop button. Pressing this one opens the possibilitiy to edit the crossfader envelope during the transition.
The flat envelope button clears the envelope. It is set to the utmost left position. Useful to start drawing your own envelopes
Clears the envelope and replaces it with a standard crossfade.
Cross correlation envelope. Fills the envelope with crossfade positions that always prefer the loudest of the two songs.
If a manual envelope is drawn, then all positions are relative towards the start of the transition. Lengthening the transition will not stretch the envelope. Moving the transition left/right will move the envelope along. That means that from a practical perspective, it is best to first align the songs, and then edit the envelope.
The crossfade envelope is orthogonal to the crossfader type. It is for instance possible to use the cross correlation envelope together with a isomorphic crossfader.