The new Word Blog over on MSDN keeps cranking out the good stuff.
Scroll down this page and read the Q and A about document types.
Q: When is a document not a document?
A: When it’s a form.
“This is another question that we spent a lot of time discussing when we started to think about building what became content controls. We didn’t just want another tool for doing online forms in Word – not only is there a great tool to do it today, but people already know how to do that in Word and don’t need another way to do the same thing.
