Bedrockgames
I post in the voice of Christopher Walken
I wouldn’t disagree with you here, although I think different games handle such flexibility in different ways. I don’t know if having a catchall phrase really helps, though, because I think it muddies the different ways that games handle this.
I think the terminology isn't as important as the idea. I see it as something that is just inherent to play because eventually the system doesn't cover something, or the system doesn't cover it in a way everyone finds satisfying, and unlike a board game, you aren't limiting to playing the rules as written. Rule zero is one expression for this I think. I usually prefer rulings over rules. But language always falls short and isn't fully comprehensive (just like rules systems!). For instance Rulings over rules can be interpreted to mean "F the rules" and I don't mean that at all. I try to run the rules as written out of fairness but I deviate when they don't work with the facts on the ground, or when the players are clearly going beyond what the rules say or they are doing something so creative and tactically sound, the rules just don't give them enough benefit mechanically. And I don't think there is a one way here. This is more just the thing I saw when I first played and something that has stuck with me as essential. But RPGs are odd and everyone seems to come to an understanding of them on their own, so I am not into prescriptive definitions of RPG terms (especially when I can see how my take on this might define out certain styles if taken too rigidly or applied universally).
The books and advice in them have been all over the map. Looking this up just to catch up and refresh for the conversation for example, I am seeing an article that contrasts Mentzer's call for fairness and involving the players with Gygax's 1E advice on the GM being sole arbiter. I tend to see the GM role as usually an important one, but I also see no reason why the thing rule zero points to (and what it points to is I think more important than rule zero itself), would be in conflict with fairness or getting player input. I tend to prioritize fairness a lot in how I approach it (and I am a very rulings over rules GM). I also am transparent about my rulings and typically ask my players if they think a particular ruling seems reasonable for what is being attempted or for what is occurring in game (this even extends to stuff like how I handle off camera battles that have relevance to what they party is doing, so it doesn't just feel like me deciding an important battle went a particular way because that is more dramatic or more what I wanted).