The first layer of the data management object hierarchy is the data access layer. Code and definitions at this level are aware of the actual source of data, whether it is a standard Data-Source object that maps to one or more database tables, or some other source. Other sources could be unmanaged data not stored in a database, or database data whose mapping to the internal data definition is complex enough that it cannot be expressed in a field mapping and database query. (The term Data-Source is used in this document to refer to an actual OpenEdge Data-Source object, as defined in Data-Source object and used throughout this book. The term data source or source of data is used more generically to refer to any source of data for the application, wherever it comes from.)