Since I felt that this is certainly not clear to everyone that uses BizTalk, some comments:

Most people are passing data to their own assemblies by passing messages, typed as an XmlDocument.  This, however does not give you access to multiple parts of that message and certainly not, to the message's context.

If you ever need to have access to the context of a message, outside of the BizTalk Server orchestration environment, you may do so by passing the message as a parameter of type Microsoft.XLANGs.BaseTypes.XLANGMessage to a method in an expression shape.