This inherited property is initialized in the InitializeTreeVertexValues method. A value of Spot.Default will cause SetPortSpots to assign a ToSpot based on the parent node's TreeVertex.Angle. If the value is other than NoSpot, it is just assigned. When Path is TreePath.Source, the port's FromSpot is set instead of the ToSpot.
This sets the RootDefaults's property of the same name.