In Curve Editor mode, Frame Value Extents adjusts the zoom factor of the Track View Key window vertically so that you can see the full height of the curve. Frame Value Extents Range also adjusts the window vertically but it zooms only to the height of the keys 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 Frame Value Extents can serve the function of resetting your view.
To zoom to the height of the keyframes that are currently visible:
The Key window magnification scales vertically to the height of the keyframes in view.
This control helps to let you focus on working with one section of your animation curve.