This kind of objects defines a vegetated surface on a terrain.
Material: undergrowth material that will be used by the area.
BlendType: Add means the area will have vegetation, sub means the area will remove it from already vegetated surface.
MaxInfluence:
FadeBorder: distance to the border of the area from which the influence will start to fade out.
(Only for circle areas)
(Only for polygon areas)
Edit Points: pressing this button will start the polygon edition mode. More on this mode
here.