ASP.NET delegates the processing of web.config data to configuration section handlers. Section handlers are declared in the web.config file using "add" directives inside the <configsections> element. Each element identifies a specific section of configuration data, and the associated System.Configuration.IConfigurationSectionHandler class that is used to process it. By inheriting from this class, you could create your own custom section handler. Note that you do not need to create your own custom section handler just to add application-specific constants to your web.config file; these constants can be added to the <appSettings> section and retrieved through the System.Configuration.ConfigurationSettings class.

sealed class ClientTargetSectionHandler : System.Configuration.IConfigurationSectionHandler {
// Public Instance Methods
   public object Create(object parent, object configContextObj, 
        System.Xml.XmlNode section);// implements System.Configuration.IConfigurationSectionHandler
}