A service interaction is automatically created for a WSDL file that contains an operation when you define the service. On the other hand, as you are creating your business process, you may know you want to use a service within the process but not know the details of the service. You can create a placeholder for the service by creating an empty service interaction (one that does not have an associated operation).
By default a service interaction does not have an associated operation. When a service interaction with no functionality (no associated service) is placed on the canvas, the icon representing the service interaction has a question mark meaning that the service interaction is not defined. Also, when you highlight the .iface file, the Dependencies view shows that the file has nothing listed for Uses or Used by.
To create a service interaction that is not associated with a WSDL file, use the New Service Interaction Wizard. To be sure that the correct operation is later associated with the service, use the To Do icon from the Simple Activities tool palette to note the required functionality.