When inspecting a material in the inspector, at the bottom we can see a material previewed on a sphere. There are two buttons above the preview, left one is for changing the object on which the material is previewed (e.g. changing a sphere into a box, etc.)
What does the right button do?
As far as I can tell it changes the lighting setup on the preview object. The “single dot” is a single light setup while “two dots” is two lights setup (the second one coming slightly from behind).