When the .NET Framework is parsing an XML stream, it needs to know how to convert the XML read from the stream into common language runtime types. The information that specifies how the .NET Framework should generate and parse an XML stream is stored in custom attributes that are located in the System.Runtime.Remoting.Metadata namespace. There are two ways to specify this information in a configuration file: either by explicitly specifying the mappings, or by specifying which object types to preload. The custom attributes are read during the preloading process and the information in them is made available to the SOAP parser.

Namespace: System.Runtime.Remoting
Assembly: mscorlib (in mscorlib.dll)
Assembly Versions: 1.0.5000.0, 2.0.0.0, 4.0.0.0