Select Under allows you to select an object which is underneath another object. This can be done in the Selector Tool by ALT-clicking on the top object. Select under is most useful when one object is completely covered by another one. Clicking on the top object selects the top object - ALT-clicking on the top object selects the object underneath.

If there are several objects covering each other, repeatedly using ALT-click selects the next object underneath the one that was last selected. When the last selected object is the one at the back, the next ALT-click will start at the top again and select the top object. For more details, see Selecting objects under objects.