The Custom Call Handler entry allows you to configure any custom handler class you create. The configuration editor creates an entry in the configuration that contains the name and the class type, plus all the attributes you enter in the EditableKeyValue Collection Editor dialog box. These values are all available to the custom handler at run time through a NameValueCollection passed to the custom class.
Only classes that implement the ICallHandler interface and carry the attribute [ConfigurationElementType(typeof(CustomCallHandlerData))] will be visible in the Type Selector dialog box and available for configuration with Enterprise Library Configuration Console or Visual Studio Configuration Editor.