Per the SOAP specification, the SOAP node determines what method to invoke based on the qualified name (QName) of the child element of the SOAP Body element.
While this may be a convention, I cannot find any mention of “invoking a method” in the spec. The SOAP spec does neither mandate nor suggest any relationship between a message and a method, which is a good thing.
I also find it very hard to agree with this assertion:
SOAP is also an application protocol, with its own set of application protocol semantics.
In my view, it may be valid to describe SOAP as a toolkit for constructing application protocols (if you are so inclined). But an application protocol in itself? I don’t think so.