This is something that Lars told me long ago.
I think mostly everybody alreay know about this.
And I don't think it is mention on the manual too.

For selecting objects on screen.
(By holding the left click of your mouse dragging it to form a red box.)

-

1,dragging from left side will select the object that comes in touch with the red box.

1,dragging from right side will select the object 'within' the red box.

This is the right side & left side difference.