clarity issues

I think it's a good start but the draft could need some more work w.r.t to clarity:

* is it allowed to define other fields ad-hoc?
* If yes, where to register these fields?
* rationale: why are we doing this? Give two, three use-cases (just makes it clearer for the user what cool things he/she could do with that format)
* what fields are a MUST?

