Only the options described below are relevant.
In the Active Tool manager, the Radius setting determines the diameter of the Live Selection brush. You can set values from 0% to 1000%.
To use this selection tool, hold down the mouse button and drag over the UV polygons you want to select.
If Only Select Visible Elements is enabled, you can only select elements which are visible, i.e., not behind other elements. For example, you can select the polygons at the front a sphere but not the polygons at the back.