Stricly speaking the External Notification plugin isn't part of the API functionality.

 

The API is for dealing with incoming calls from remote servers to do X within Jomres, whereas the External Notification plugin is designed to allow you to automatically update a remote site when a booking is taken. You can configure the method that the system uses to send the information, which variables to send and what name you need to use for each of these variables, and finally the url of the remote site.

 

More information on how the plugin works can be found here.