Layers can have a stacking order that allows a virtual third z-dimension to be defined.
Using this, layers can appear on top of other layers. Layers can be nested and moved in a dynamic fashion via scripting. It overrides the default behavior of placing new layers on top of all existing layers. References to layers not yet defined result in default layer creation behavior - i.e. the new layer will be placed on top of all existing layers. Default behavior for a layer is to be transparent.