The buttons 4,3,2 and 1 on the left used to generate Dark Luminosity masks that are based on pixel luminance.
The buttons 1,2,3 and 4 on the right used to generate Light Luminosity masks.
For Example: Clicking on 4 of the Darks will result in selection of the darkest area of an image and clicking on 4 of the Lights will result in selection of the lightest area of an image.
See the images below:
Clicking on the Midtones button will result in selection of Midtones areas of an image. Similarly click the Highlights button to automatically select Highlights or click on Shadows button to select Shadows.
To select a specific color or color range of an image click on the Color button.
To reset selection - simply click on Deselect button.