The term z-order refers to the order of the layers of the zones within a layout.   The lowest number z-order is always furthest to the back and any zone with a high z-order number will be shown "on top" of zone with a lower z-order number.
For example. A zone with a z-order of 1 will be "behind" a zone with a z-order of 2.

The z-order is only relevant of there are overlapping zones in the layout.

The layout has three separate layers and a z-order within each layer.

  • Background - There can only be one "background" zone. This zone will always be the furthest back and all other zones will be on top of the back ground zone, regardless of the z-order value of the background zone.
  • Foreground - The foreground layer is the default layer for all new zones. Any foreground layer will show on top of the background layer and the z-order of the zones within the foreground will be compared to all other zones in the foreground. 
  • Overlay - Any zone set to overlay will show on top of the foreground and background layers regardless of the comparative z-order. There can be multiple overlay zones. All zone within the overlay layer will respect the z-order of all other zones within the overlay layer.  

The image below shows two overlapping zones, the "purple" zone has a high z-order number than the "blue" zone, so it will be on top.