Yvonne,
SAP NetWeaver Application Server, expose services. Every interface is a service on the server. When those services are called, SAP Application Server performs the stipulated action (execute the program which is designed to handle those requests).
What does the service accepts? What does the service return? Where does the service reside on server?
This type of details are defined in WSDL. WSDL is an agreement between requester and server, 'what to send to server and what to expect back', 'which program to execute on server'.
"The Web Services Description Language (WSDL) is an XML-based interface definition language that is used for describing the functionality offered by a web service."
Service interface in ESR is a WSDL (without Concrete Section 'which program to execute when service is called'). When configuration in ID is done 'Sender Agreement' will have the WSDL (with Concrete Section).
It is not mandatory for requester or the server, to honor the WSDL. WSDL can say something, but requester can send what ever he wants. WSDL can say something, but server can do what ever is written in the program on server (Concrete Section 'binding, service, port, program').
How to send any data (even binary) through XI, without using the Integration Repository. WSDL can say it accepts XML, requester can send image, pdf, txt, video. Using Java mapping OR no mapping, PI can handle image, pdf and provide response OR perform some action(if WSDL validation is enabled, it will fail).
In short, SI can say anything. Request can send anything. But still request response can be successful, 'have to program on server accordingly'.