External Apps allows to set how external applications interact with SSP. These external apps are not part of the main core of SSP, but use the SSP WebService to start tickets in selected processes. This section does not explain you how to build applications like this, but does explain how the fields are defined that enter SSP. Any external app will basically replace a Form in SSP, so it sends over data to the WorkFlow engine, to start off one or more processes. The WebStore is considered an external application, for example.
The External Apps screen is opened by clicking on the External Appsbutton in the Admin Panel.