The order in the z buffer determines the order in which overlays clip each other. Overlays are assumed to be on top of all other screen components. Overlays that do not have a specified z-order behave in unpredictable ways when overlaying the same area on the primary surface. Direct3D Retained-Mode does not sort overlays if you do not have a z buffer. Overlays without a specified z-order are assumed to have a z-order of 0, and will appear in the order they are rendered. The possible z-order of overlays ranges from 0, which is just on top of the primary surface, to 4 billion, which is as close to the viewer as possible. An overlay with a z-order of 2 would obscure an overlay with a z-order of 1. No two overlays can have the same z-order.