[Attribute Name] USAGE [Type] String [#] Exactly One [Default] Default [Description] 
The default usage of variables is for specification of input and output parameters. Variables can also be used internally within WIDL, as well as to pass header information in an HTTP request. For instance, using internal variables a portion of a service's URL or a pattern for matching within an object reference can be specified as a variable that is part of an input binding.