Store schema definition language (SSDL) is an XML-based language that describes the storage model of an Entity Framework application.
In an Entity Framework application, storage model metadata is loaded from a .ssdl file (written in SSDL) into an instance of the System.Data.Metadata.Edm.StoreItemCollection and is accessible by using methods in the System.Data.Metadata.Edm.MetadataWorkspace class. The Entity Framework uses storage model metadata to translate queries against the conceptual model to store-specific commands.
The Entity Data Model Designer (Entity Designer) stores storage model information in an .edmx file at design time. At build time the Entity Designer uses information in an .edmx file to create the .ssdl file that is needed by the Entity Framework at runtime. For more information, see .edmx File Overview.
The XML schema for the .ssdl file can be found in the XML\Schemas folder of your Visual Studio installation directory (for example, C:\Program Files\Microsoft Visual Studio 10.0\Xml\Schemas). The schema for applications that target the .NET Framework version 4 is defined in the System.Data.Resources.SSDLSchema_2.xsd file. The schema for applications that target the .NET Framework version 3.5 SP1 is defined in the System.Data.Resources.SSDLSchema.xsd file.
Versions of SSDL are differentiated by XML namespaces. The http://schemas.microsoft.com/ado/2009/02/edm/ssdl namespace is for storage models in applications that target the .NET Framework 4. The http://schemas.microsoft.com/ado/2006/04/edm/ssdl namespace is for storage models in applications that target the .NET Framework 3.5 SP1.