The topic has also come up in a discussion thread at Modern Analyst which explores what the difference between functional specifications and business requirements specications is. (here)
The three levels are typically called;
Complementing functional requirements there are non-functional requirements. As a result, in some circles the functional requirements are called user requirements. To me this is a neat label because it keeps the focus on users and is a good term for addressing non-technical solutions.
So we can look at the hierarchy as;
To try to explain the relationship of these hierarchies I have put together a Venn diagram. I am interested in your comments, either here on in the Modern Analyst forum thread.
And after posting the diagram above I realise I have not labelled the "User requirements" - but I don't have time to redo the picture; so be aware it's the second level.