MACROMEDIA FLASH 8-COMPONENTS LANGUAGE Reference page 500

Test the SWF file. When you click the top button, the other button changes depth and moves
to the front, and the Output panel displays that button's depth. The values are 20, then 40,
then 60, incremented by 20 each time you click.
If you use DepthManager with movie clip instances instead of component instances, you
may need to add a UI component to your library (if one isn't already there) for
DepthManager to operate properly. DepthManager requires a component on the Stage
or in the library to function.
For more information about depth and stacking order, see "Determining the next highest
available depth" in Learning ActionScript 2.0 in Flash.
498 DepthManager class