public TerrainLayerSmoothnessSource smoothnessSource { get; set; }

Used for choosing where smoothness value comes from when TerrainLayer.diffuseTexture has an alpha channel. Setting to TerrainLayerSmoothnessSource.Constant results in using the smoothness value set in TerrainLayer.smoothness. Setting to TerrainLayerSmoothnessSource.DiffuseAlphaChannel results in using smoothness value set from alpha channel of TerrainLayer.diffuseTexture.