All the data exposed by the XData server is specified by an Entity Model represented by the class TXDataAureliusModel (declared in unit XData.AureliusModel). The Entity Model describes all the types (classes) and properties provided by the server, among other info.
Although XData uses Aurelius mapping to know all the classes and properties of the data objects, the Entity Model is used as an extra, intermediate metadata layer between XData and Aurelius. XData uses the Entity Model as the "official" metadata, and in turn, the Entity Model contains the links to the Aurelius mapping. This allows additional flexibility, like for example, having a property name in XData to be different than the property name in the mapped Aurelius class, although both point to the same property.
The entity model in summary, describes the published entities (through entity sets) and the service operations available.
The following topics explain in more details how XData uses the entity model.