The BizTalk Server 2006 bindings file format is documented here, but the XSD description of the bindings file is not included in the build. If you are interested in having this XSD, you can produce it via the following command:
xsd.exe “C:\Program Files\Microsoft BizTalk Server 2006\Microsoft.BizTalk.Deployment.dll” /type:BindingInfo
The xsd.exe tool comes with .NET Framework SDK.