I would generally view "the fiction" as a specific table instantiation of a "setting".
Forgotten Realms is a setting. My D&D table running a game in the Forgotten Realms creates a specific fiction.
For reasons you can probably infer (based on my posting history), I'm a little sceptical about the "reality" or "ontological heft" of a setting like (say) the Forgotten Realms, or Greyhawk, as having "definition" and content - for RPG purposes - independent of some particular instantiation in someone or other's use of it in play.
To try and be a bit clearer: I don't mean to say that we can't talk about (eg) what Ed Greenwood wrote in a book. But suppose Ed Greenwood once wrote a book in which one character said to another "Hey, watch out for that magic-user - if you're not careful, she'll fireball you!": it doesn't follow that, just because I choose to use that book as a basis and framework for my own play, that I'm committing to game elements like
the magic-user class and
the fireball spell being discrete and identifiable elements of the fiction I'm imagining and sharing with my fellow players.
But the preceding is something tangential to this thread, I think. Though I'm very happy to discuss it further if it's of interest!
"Diegetic", as the jargon has been adopted, is discussing something more specific than simply "is the mechanic representing a concept within the fiction".
Well, this did come up in the thread. And the OP indicated a usage of the term "diegetic" that fit with what I suggested, not what you've said here:
When asking if a game mechanic is diegetic we're not asking if the mechanic itself exists in the game world, but whether it has a referent in the game world.
I think a series of posts in the 70s expressed a similar view.
It's describing whether the mechanical element exists as a descriptor that can be used for in-character discussion and communication.
<snip>
The concept of "class", and whether or not class is something identifiable and recognizable within the fiction, is where the term "diegetic" is doing the bulk of its work.
For this, I think I still prefer
is in the fiction or
is an element of the fiction. As per my post 151 upthread, and also this post from earlier upthread:
a lot of the mechanics that characterise classes, in modern D&D at least, are really only metagame ones. For instance, in the metagame (that is, the mechanical play of the game) there's a difference between two attacks vs one target for (say) d6 damage, and one attack vs that target for (say) d12+1 damage. But in the fiction - where the action economy, rates of attack, hit point loss, etc are not literal elements of that fiction - those do things could be very much the same.
The reason I've requoted that is to foreground my earlier remark about which mechanics are metagame and which are not. I think a lot of mechanics that are often not recognised as metagame actually
are - in the sense that they don't particularly represent or model anything in the fiction - and once we recognise this it should shape our thinking about what other stuff is part of the fiction.
For instance, if we think about a fighter vs a rogue in the fiction, how do they differ? First, the difference is most noticeable - and, perhaps,
only noticeable - in the context of
how they fight. Because the way that skills are resolved in 5e D&D isn't mechanically "tight" enough to extrapolate anything concrete into the fiction. In a fight, a fighter likely wears heavier armour than a rogue, and perhaps also wields a heavier weapon. The rogue is more dangerous when fighting alongside a friend, or when attacking from some sort of cover/surprise.
And so the question
are rogues and fighters real things in the fiction becomes something like
in the fiction, is there a difference between an opportunistic skirmisher and a hold-the-line heavy infantry-type? To which the answer is probably "Yeah, I guess so" but I don't think that takes us very far. The differences between the two classes are much more mechanically intricate than this, but those differences - that define the classes as components of game play - don't manifest in the fiction.
(And the mechanical viability of DEX-based fighters using hand crossbows or whatever only reinforce the preceding point.)