Zoom Value Extents is a flyout containing the Zoom Value Extents and Zoom Value Extents Range buttons.
In Function Curves mode, Zoom Value Extents adjusts the size of the Track View Key window vertically so that you can see the full height of the curve. Zoom Value Extents Range also adjusts the window vertically but it zooms only to the height of the keyframes that are currently in view.
To zoom to the height of the curves:
The Key window scales vertically to display the function curves.
This control is particularly useful if you have zoomed in and need to see all the keys again. There is no undo for view changes that occur in the Key window, so the Zoom Extents button can serve the function of resetting your view.
To zoom to the height of the keyframes that are currently visible: