To consume a SOAP web service, developers simply provide the location of the WSDL (Web Services Description Language) file. The OutSystems IDE inspects the WSDL and generates everything needed to invoke the web service methods.
To consume a REST API, a developer needs to provide the service end-point URL, and an example of the service request and response in JSON. Most services include a JSON sample request and response in their documentation and it can be pasted into the OutSystems editor. OutSystems generates everything needed to invoke the REST service.
After OutSystems has generated the service methods and data structures, invoking the service is seamless. From a developer's perspective, there is no difference between invoking a system method, or a method exposed by an external service.