Prior to exploring the details of any message implementation, there is usually a significant benefit in having clarified what data is required to support the business need.
These models may also serve as the medium between business and IT and eliminate sources of misunderstanding. Secondly these models may easily be transferred into machine readable formats like e.g. XML Schemas (XSD files) needed to process regular XML files.
Please check the "Models" for an example and / or suggestion to how this may look.