Defines the attached behavior that keeps the items of the Selector host control in synchronization with the IRegion. This behavior also makes sure that, if you activate a view in a region, the SelectedItem is set. If you set the SelectedItem or SelectedItems (ListBox) then this behavior will also call Activate on the selected items. When calling Activate on a view, you can only select a single active view at a time. By setting the SelectedItems property of a listbox, you can set multiple views to active.