I thought there is but one document outline (per document). However for syndication reasons, your articles and maybe also sections? can be thought of as separate, smaller documents themselves. That is, they can have a document-like structure (<header>, <hgroup>, <footer>, <aside> and a body of content in the middle, though you would not add the "main" role to that body since articles are indeed not really separate documents).
I also find it rather retarded how they (WGs) decide some stuff: they add this "hidden" attribute that does nothing special (and will absolutely be abused by people who don't have any clue what it's for, esp since it doesn't really have any use cases), yet want to remove half-broken stuff from HTML4 without at least trying to replace them with something similar that does work (longdesc, table summary). The arguments against those last two could give a kung-fu kick to "hidden" attribute and send it flying into deep space, never to be heard from again.