When you mouseover an object it gains a green border.
When you click on it, you "select" it, represented by a blue border.
You can then use mousewheel to select objects below the mouse (mousewheel up, or "zoom"), and to reverse the selection (mousewheel down), all the way to deselecting completely.
In Type 1, you can move your mouse without breaking selection. This means, for instance, you can select an object, move your mouse to a completely different object and mousewheel up to select this new object.
Order of selection is preserved correctly irrespective of mouse movement.