A SOAP node is a SOAP web service. Many SOAP nodes can be spread over many computers or can be deployed all at one and the same computer.
In your example it is appropriate to say that:
Web service A sends a request to web service C and the request passes the intermediary web service B. This means that the web service B does not process the request, but rather passes it on. Web service B may add or remove SOAP header blocks, but that is another story.