When selection is performed, the selected item is the one that is most "in...

When selection is performed, the selected item is the one that is most "in front" (or closer to the viewer).  This is performed using the screen depth.  There may be multiple items under the cursor and in this case, the selected item changed only when the depth of an item drawn was less than the last item found under the cursor while drawing.  This depth test has been changed to "less than or equal to" since some items, such as annotations, are drawn at the same depth so that the item last drawn, that is drawn on top of all other items, is selected.