Brian Attebery described genres, particularly fantasy, as "fuzzy sets" - not defined as much by a boundary as to what is or is not within the genre, but by relationship to a center point, which may be represented by a kind of "ur-text" (or set of core texts; e.g. LotR for epic fantasy, Conan for S&S).
D&D is tricky, because it is almost its own family of genres. We could say that the ur-text is early D&D as a whole, both OD&D, Basic, and 1E -- how D&D was originally played and envisioned. But it has expanded quite a bit beyond that, and includes a variety of secondary and related fuzzy sets. And really, every player has their own ur-text, whether a particular edition or campaign setting.
In truth, I see this as the underlying purpose and even beauty of campaign settings, that I think is implicit in their creation and use: each one creates its own sub-genre, its own fuzzy set. But I also find that this sometimes clashes with what we could call "canonical purism," that is, adherence to whatever the latest version of canon is. This tension pops up in different ways - perhaps the insistence by some that a particular setting must include (or exclude) certain elements of canon. Or the old classic "I want to play X," player says; DM replies, "Sorry, they don't exist in this world." "But they're in the rules!" Etc.
I think the idea of D&D as a toolbox is useful here, which implies that you don't have to use all the tools (I mean, you can't possibly include everything, except in potentia), and perhaps more importantly, you can use the tools as you want to. Screwdrivers are made to screw (heh), but you can also use them to pry open paint cans or scratch an itch. If you're designing a campaign world, as the DM, you may consider which races to include, whether they fit your core themes and vibe, and have to make choices: Do I exclude this race, or do I alter it to fit the vibe I'm going for? In considering player preferences, you then consider choices and compromises. For instance, if dragonborn don't fit the vibe you're going for but one of your players loves them, you then have to decide how to serve two masters. Maybe dragonborn don't exist in your world, but except as a random mutation or as the result of magic, or maybe the PC was transported from another world and is a unique specimen. And so forth.
In the larger picture, we can imagine a set of super-imposed, nested fuzzy sets, interacting with each other. You have "fantasy" and then you have "the D&D toolbox," and then you might have a specific campaign world, and then you have your version of it (or a homebrew), for which you might create an actual ur-text: a campaign document that describes the center around which everything relates.
Meaning, the toolbox and fuzzy set allow individual DMs--as campaign world creators--to envision their own "ur-text," and how different elements of the toolbox fit in, whether they fit at all, or how they can be made to fit.
