The item represented by obj has been dropped onto the control.
If the TreeView DragReceiveBehavior is set to 1 (Directory nodes) then text bellow applies, otherwise all nodes can get DropObject event.
When DragReceiveBehavior = 1:
The picture bellow will show the possible drop zones
If within the red rectangle then the node parameter is nil as it is the Root node that the drag item is over. If within the green rectangle then the node parameter will return the Test4 node. If within the orange rectangle then the node parameter will return the Test4b node. If within blue or light blue then the node parameter will return those nodes.