If AutoDock is true (the default), when a dockable object is dragged over the control bar, the control bar temporarily docks the object so that the user can see the impact of the drag operation. That is, the user can see the dock position and how other docked controls will be repositioned to make room. When the user releases the mouse, the dragged object is docked at the current spot, and any temporary dockings are canceled. However, if the user cancels the drag-and-dock operation, the dragged object reverts to its last temporary dock site rather than to its location at the start of the drag operation. 

If AutoDock is false, the control bar does not temporarily dock dragged objects. Instead, the user only sees the dragging rectangle as it passes over the control bar, with no feedback as to the adjustments the control bar would make if the dragged object were released. 

DockClients 

DockSite 

OnDockOver