2. If you order this way, you are absolutely certain, that you successfully created a valid document in your partners system, what will be executed as an internal document. It will be not lost, overwritten, or misinterpreted by some interface.
Conclusion: If I would plan system architecture today, if possible, I would use an always actual online service instead of a conventional system with redundant data in files or tables. Of course SOA has many other parts and details – you can check out the references - but I find it very important to understand the essence, that everything is based on services.
SOA on wikipedia.org
Enterprise SOA on sap.com
SOA defined by XML.com
Amazon.com is a SOA platform!
Look at the other side as well (contra SOA)