Each third party service represents a remote system that your application can communicate with directly from the page. Depending on the Submission Method of the link calling the third party service, this could be an asynchronous call to update part of the page, or a form submission or URL call to return a completely new page.
The Third Party Service Details
panel (shown when one is selected on the diagram) allows the entry of a Location URL. This is the URL that will be called to access the remote system.
As mentioned above, the response from third party services will be assumed to be HTML pages (or fragments) and used to directly render to the page. If you need to manipulate the response received form the service before rendering the page, you may want to use a standard Controller to communicate with the remote service. This then provides the full data manipulation capabilities enabling message repurposing as needed before rendering.