The secondary selection is another way of selecting text using X. It does not use point or the mark, so you can use it to kill text without setting point or the mark.
mouse-set-secondary). The highlighting appears and changes as you drag. If you move the mouse off the top or bottom of the window while dragging, the window scrolls at a steady rate until you move the mouse back into the window. This way, you can mark regions that don't fit entirely on the screen.
mouse-secondary-save-then-kill). A second click at the same place kills the secondary selection just made.
mouse-kill-secondary). This places point at the end of the yanked text.
Double or triple clicking of M-Mouse-1 operates on words and lines, much like Mouse-1.
mouse-yank-at-point is non-
yanks at point. Then it does not matter precisely where you click; all
that matters is which window you click on. See section Mouse Commands for Editing.