Selectable is an option for notebooks that determines if the window for a notebook should become selected when you click it.
The default setting is Selectable True.
With the setting Selectable False, a notebook window does not become selected by your clicking any point inside the notebook.
Selectable only affects the selection of the contents of a window. The notebook can still be selected by clicking its title bar. The related option WindowClickSelect affects selection of the window as a whole.