Logically, a document page consists of three transparent layers: Main, Background, and Overlay. Main is the default layer, and all block flow rendering goes into that layer. Direct rendering can be done into either of the three layers, the specific layer being determined by the PageLayer property.
All rendering into the main layer overlays the Background layer, and all rendering into the Overlay layer overlays the Main layer. The Background layer could be used to print watermarks, for example, while the Overlay layer could be used for overprints.
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