I find myself involved in discussions frequently where one person will claim that a piece of text in a rulebook is "just fluff" or "pure flavor" and can be "refluffed" by the players with little or no DM oversight, while others are "mechanics" or "crunch" that requires a house-rule to change.
Specifically, the things cited as "fluff" are those that do not have an attached dice roll or explicit benefit/penalty listed. Things like the descriptions of classes, feats, things like "Druids won’t wear metal armor." On the other hand, "mechanics" or "crunch" includes rule text that has direct, explicit application to the game world, especially when expressed in pseudo-mathematical form (such as dice rolls, damage, or bonuses and penalties).
I know that 4e had a specific rule that said that descriptive text in italics in an ability entry or elsewhere was just an example (ie fluff) and could be changed at will. But I’m pretty sure that this distinction pre-dates 4e–I remember hearing it invoked to justify taking prestige classes (3.5e) completely out of context and apply them without satisfying the "fluff" prerequisites (such as meeting/being trained by someone of the appropriate faction).
I’m interested to know where (and in what context) this distinction first (or dominantly) arose. More than that, I’m interested in finding examples of where this distinction has direct rules support. So far, 4e D&D is the one I know of. Citations to rulebooks would be wonderful. This is not D&D specific, although most of my knowledge is of D&D’s history.