Differences between Message Flow and Control Flow in Xlang (or BPEL4WS)???

In XLANG, information flow is distinguished into the following three categories: Message Flow, Data Flow, and Control Flow.

So what are the differences between Message Flow and Control Flow? It seems to me that they are the same.

In BPEL4WS, they even have Control Flow, Message Flow, Data Flow, and Transaction Flow.
What are the differences among Control Flow, Message Flow, and Transaction Flow?

I am confused by these terms....

Thank you!

Jim