I see much underlying unity across these questions, and I hope you can too. The central concept is situation in play, which certainly needs more practical clarity among us as it relates first to texts, and also to straightforward behavior. Oddly, too, it matches with the final question concerning explicit role-playing references in music, as the general failure of situation in hobby play is almost perfectly aligned in time with the more visible presence of role-playing in various forms of rock-and-roll. I doubt there’s a lot of meaning to be discerned in that simultaneity, but it might not be entirely trivial either.
In my reply to Pablo, I forgot to say a bit about Traveller situations, as a perfect example of providing “bits” of the known types of fiction/situations which were to be formalized by procedure, but understood by all to be only components of a bigger picture or understood context in which we played.
In terms of how Claudio asked almost the same question, the text of original Traveller has no explanatory content at all. It was written by people who understood X (which is to say, Dumarest of Terra and a very few related works) for people who already knew and liked it. What’s to explain, or even to mention? Here are some procedures that work nicely to snap together many other decisions and standards for content, which you already know.
Regarding Sean’s question about music, I also thought about role-playing’s explicit appearance generationally. It would be my age-group, born in the early-mid 1960s, who’d be of the age to put D&D into music publication, in our early twenties, so 1987-ish (the dates so far identified) would be exactly right.