7. The Menus / 7.6 The Tool Menu
HDR images can contain color values smaller than 0% and larger than 100%, but only the range from 0% to 100% is visible.
If you want to modify the visible range of an HDR image, the normal Levels function is not well-suited, because it will show you only the existing values from 0% to 100%, Optimize HDR instead will show you the full spectrum of values, so you can easily define the visible range. Additionally it will show you the minimum and maximum values of the image.