Message is a
collection of parts
The message is not a real thing by itself, but just a bag of things
Each part represents
one thing to be sent or received
Dont just think of RPC
parameters a medical record being sent to a
doctor may consist of some XML document (contained
in the SOAP envelope) and lots of other stuff like XRay images etc. as attachments. Each such thing would
be modeled as a part.
In other scenarios it may be multiple XML documents or elements from different namespaces (purchase order +
vcard)