Selecting the button will auto-fit all of the points into the Envelope Display.
This slider controls the how much of the Envelope Display is seen. At the minimum value it will display a small part of the first segment and a maximum setting all 100 bars are visible.
If your mouse has a scroll wheel, you can use it to zoom. Mouse-over the envelope display and move your scroll wheel to zoom.
NOTE: If the Envelopes contour is very short, less than 1/4 of a bar, the Zoom slider will not be able to fill the Envelope Display area with the full Envelope.