So, this position can lead to somewhat counter-intuitive results - specifically, that if you have two different texts expressing the exact same mechanics in different ways, you have two different systems. This happens these days when you have games with SRDs. To say that the Fate Core rulebook and the Fate Core SRD are different systems is not clearly correct.
Interesting, and I think the problem here is that I don't have a good word for the distinctions I'm wanting to make.
In another thread, the idea of how much you needed to transform the rules of a game before it was recognizably a different game or a
different system (to which many different games could belong). In that thread, to simplify, I think everyone was mostly talking about a
game as if it was defined by the rules, something that I elsewhere deny because what happens at the table is so much more than the written rules. But there is also a sense in which I agree there is a correspondence between (a closely related family of) the rules and
a game (say 5e D&D), and a less closely but a still related family of rules and
a system (say D20). Certainly when I talk about systems in that sense, I thinking more in terms of something broadly shared.
But here I was using system in a slightly different way, and I think I have an overabundence of definitions I'm not sure how to clean up.
On the one hand, for some definition of system, we'd expect to reasonably describe the Fate Core rulebook and the Fate Core SRD as belonging to the same system, and players of one to recognize the process of play and rulings from one to be rather closely related to the other. And, at the same time, I think that it is true that using the Fate Core rulebook versus the Fate Core SRD is likely to lead to slightly different collection of rulings and understandings, which are in effect going to be a different
game (by a slightly different definition of game than I used when I used giving 5e D&D as an example). (I'm not really knowledgeable enough about either to say this for certain about FATE, but I do know that this is true of using the D&D Player's Handbook versus the associated 3.X SRD.) That is to say each unique collection of processes of play and rules is its own distinct game, even if they are related through the partially transmitted (from the text) experience of being the 'game' we call 5e D&D.
I welcome clearer taxonomy terms for these distinctions.
The community already has concepts for Rules as Written and Rules as Intended. I think those are highly relevant to the discussion.
Maybe, but "Rules as Intended" is going to prove to be a really awkward concept in this discussion. For example, going back to the PC showing a bit of ankle to the guard in 1e AD&D, you might think that the rules as intended relating to Encounter Reaction Rolls and Parley are pretty clear, but there is an oddity that earlier in the rules, Gygax as an aside tells the reader that he doesn't actually use the rules that are in the book regarding Encounter Reaction Rolls. Instead, he gives a very brief summary of a system he's using at that time where the reaction is determined by a draw from a deck of cards, and how that system would work with Charisma or other subsystems like the NPC personality tables isn't made clear (since you can't easily apply percentage modifiers to the draw of cards). So what does it say about how the writer intends you to use the rules when he himself doesn't really use them, but has a largely undocumented homebrew system?
I think that rather than worrying about Gygax's intentions and mental state, it's better to talk about the participants intentions and mental state, which would lead us to the idea of 'Rules as Understood' or even 'Rules as Applied'. Whether or not the table actually correctly divines what game the writer wanted them to play is less interesting than what game the participants decided that they should play and why. This also might lead into a discussion as to whether it's an allowable process of play for a player to cite the rules in protest to a ruling. That is, what do you appeal to when you as a player think that the GM has got it wrong, and does the game actually validate protesting as something you are allowed to do. (Paranoia the RPG would be a case in point.) When 'rules layering' happens in my experience, it's frequently not at all clear from the text what was intended by the writer.