ToolWindow is the container control that hosts all panes when in floating state. Every time you drag a pane and undock it, a new instance of the class Telerik.Windows.Controls.Docking.ToolWindow is created to host that pane for you.
The implementation of the RadDocking never requires you to work with ToolWindows directly, rather than with the pane hosted inside. For example, to make a ToolWindow not closable just set the property CanUserClose of the hosted pane to False.
On the snapshot below you can see two ToolWindows with pane inside of each one.
If you need to find out how to declare such a floating structure take a look at the Floating Panes topic.