SOAP is like a set of Russian dolls in its layers of encapsulation. There are the HTTP headers, then the SOAP envelope, and then the SOAP body. Each layer provides metadata for the immediately enclosed layer.
Metadata is information that helps an application make sense of the important data being transported. For instance, the HTTP header specifies the character encoding as metadata for the HTTP body. This makes sure that the application properly translates the stream of bits into a series of characters.
The outermost portion of the HTTP body is the SOAP envelope. This contains the SOAP-ENV:encodingStyle attribute which specifies the structure of the actual request. This metadata tells the application how to make sense of the XML elements and attributes within the body, which make up the actual request.