The following documents explain the ESB Envelope in some detail, and explains its positive effect on a services based archtiecture.
This is the whitepaper on ESBEnvelopes.
ESBEnvelopes.mht
This discusses how to send an ESBEnvelope to an ESB.NET Server.
SendingRequestsToESB.NET.mht
Other references on Variable Content Containers
http://www.xfront.com/BestPracticesHomepage.html
http://www.xfront.com/HermaphroditeTypes.zip
and XML best practices...
http://extensibilitymanifesto.org/The_eXtensibility_Manifesto.pdf