Layers
Layers contain the rules of how spatial data is rendered on a map, whether that data be vector features, grid or raster images, labels or dynamic generated data such as pie or bar charts. Thematic rules are typically used to determine the map styles in a layer. This dynamically connects the map visualization with the runtime data associated with the map elements.
The bottommost layer is drawn first and the topmost layer drawn last. Layers containing features that would obscure the features of other layers should be placed lower. For example, a layer of boundary regions should be placed beneath a layer of points.