Here are the abbreviations, you'll find a full list on
this page.
LButton - the left mouse button
RButton - the right mouse button
MButton - the middle or wheel mouse button
WheelDown - this is equivalent to rotating the mouse wheel down (toward you)
WheelUp - the opposite of the above
WheelLeft and WheelRight
XButton1 - a button that appears only on certain mice
XButton2 - a button that appears only on certain mice