Range

The Range combo box influences all sliders on the Adjust tab sheet. It can be used to limit the brightness range of the effects. Here are some images that demonstrate the effect with the Flat slider set to 8:

Original Texture | Flat Blur with Range set to "All" |
Flat Blur with Range set to "Height & Mids" | Flat Blur with Range set to "Mids & Depths" | Flat Blur with Range set to "Heights & Depths"
Flat Blur with Range set to "Heights" | Flat Blur with Range set to "Mids" | Flat Blur with Range set to "Depths"
Flat Blur with Range set to "Upper" | Flat Blur with Range set to "Lower" |

The All setting affects the whole brightness range, the "Heights & Mids" option affects only the brightest and medium bright pixels, the "Mids & Depths" option affect only the medium bright and dark pixels and the "Heights & Depths" option affects only the brightest and the darkest pixels. The "Upper" setting affects only the bright pixel while the "Lower" option influences only the dark pixels.