All right, just one diagram...
This is an out-of-universe compromise theory between current Multiverse Theory, your Inspiration Worlds, and the Sakai/doctorlit 'all worlds is reals' idea. Essentially, it goes like this:
There is the Real World (black). We live in it. To the best of our knowledge, it is the only universe that actually exists, with the others all being fictional constructs. Yuo can believe whatever you like, but at the end of the day, this universe is the only one that can be scientifically verified to exist. It's also entirely unknown (and unknowable) to the PPC. They can't interact with it, because any interaction would necessarily involve the 'Real World' being written - which would make it actually World One.
World One (blue) is a fictionalised version of our world. Specifically, it's the one we as PPC authors have created - one identical to ours in every way except for the fact that plotholes occasionally take people to HQ, agents stop by for visits, fangirls can sometimes break into canon worlds, and there's a PPC city in New Caledonia. This is the universe the Flowers come from, along with most agents. As far as the Flowers are aware, it's the only Atomic world.
Separate from World One, and untouched by the plotholes of the Cascade - in fact, entirely inaccessible to the PPC - are the Inspiration Worlds (light blue). These are also Atomic worlds (though the term is inaccurate - see below), and have their own independent existence. Through some means unexplained by PPC science - because they don't know it exists - they can form links with World One, allowing authors to create stories.
Those stories are canons (red). They appear when they're written, and it's possible to create multiple canons out of a single Inspiration World: Tolkien's Book of Lost Tales is a distinct, cohesive canon which is utterly irreconcilable with LotR, yet clearly comes from the same Inspiration World.
Then there is fanfic. Fanfic is created in much the same way as canons, except that instead of some mystic (pun intended) link, the World One authors are inspired by reading/watching/hearing about the canon. There's a purely artificial distinction between 'fanfic' (green) and 'secondary canon' (orange) - the latter is pretty much fanfic which is acknowledged as official. The Movies fall neatly into this spot.
Obviously, secondary canons can also have fanfic written about them, and even secondary canons of their own, such as the games which spin off and alter the Movieverse. But then, so can fanfics! OFUs are all fanfic of both OFUM and their host canon. It's absolutely possible for a fanfic to have two or more inspiration (lowercase I) canons.
Tangled up in all of this is feedback. Everything feeds back into the world that inspired it, and the world that created it. World One's J.R.R. Tolkien became more hobbitlike after he started writing about hobbits. New Zealand has practically turned into Movieverse Middle-earth in places. The Myst idea of stories slightly altering their inspirations indicates there's feedback from canons to Inspiration Worlds - and there's no reason that can't include the idea that agents from canons, while technically recruited through red or green worlds, are 'echos' of a light blue character who is drawn through into the PPC and 'merged' - for want of a better word - into a single 'real' person. That would allow people from fictional worlds to be 'real' without contradicting established canon.
Not shown on the diagram is the heirarchy of Word-detection. It runs like this:
-Agents can see the Words in fanfic (green, potentially orange though I don't think that's been attested).
-Agents generally can't see the Words in canons (red), though Flowers can sense them.
--Red, orange, and green, therefore, are all classed as Word Worlds by the Flowers - because they can sense the Words there.
-Not even Flowers can feel the Words in World One (blue), even though it is a written world. The same would apply to Inspiration Worlds (light blue) if they ever accessed them.
--Blue and light blue are therefore classed as Atomic Worlds by the Flowers. They're wrong, but it's their mistake. It probably has something to do with that being their 'native' level of reality.
-There are no Words in the Real World (black) - as far as we know! To the best of our knowledge, this is the only true Atomic World. The Flowers, of course, know nothing of its existence.
Though actually... the non-interaction of World One and the Real World may not be quite that absolute. Swan's Egg features interaction which didn't involve fictionalising the Real World - because the only interaction between the fictional PPC agents and us took place right here on the Board, in the form of written posts. Of course, the posts for the PPC were written by me - but they're a written world anyway, so they were always going to be.
RPs wouldn't count, because they involve creating fictionalised personas. Agent LJs might be interaction, if authors reply to them as themselves, rather than in-character. Agent Lou talks directly to me-as-the-narrator, without me making a fictionalised appearance. So there are ways to do it. But anything that involves writing the Real World makes it, instantly, into World One.
hS