APEX_WEB_SERVICE API to invoke a Web service and examine the response anywhere you can use PL/SQL in Application Express.
The following are examples of when you might use the
When you want to invoke a Web service by using an On Demand Process using Ajax.
When you want to invoke a Web service as part of an Authentication Scheme.
When you need to pass a large binary parameter to a Web service that is base64 encoded.
When you want to invoke a Web service as part of a validation.