RootObject specifies the object that will be used as a root objects in the returned xaml.
If null or empty the last read object will be set as root object.
With setting the RootObject a custom object can be used as a root object. For example "UserControl" or "Window".
It is also possible to specify more then one root object - for example "UserControl;Grid;Grid". This will use UserControl as a root object and than add Grid as its child. The read objects will be then added to the Grid.