Microsoft Wireless Laser Mouse Microsoft Wireless Notebook Laser Mouse 6000 Side Button's Function?

Microsoft Wireless Notebook Laser Mouse 6000 side button's function? - microsoft wireless laser mouse

What is the meaning of any side key, which is on the Microsoft Wireless Notebook Optical Mouse 6000? I know that the right mouse button, left click, scrolling, and press the Return key to open the battery something. But the point of the button on the left side?