Fluff and Metaphysic's

The Building Blocks From Which Worlds Are Made.

To understand how Fragile Alliances works one must first understand the difference between 'Fluff' and 'Metaphysics' as those are the two over-arching types of information from which the world is built and described.

Fluff

'Fluff' is the cool stuff that gives the world its richness and flavour. It's what the people who live in the world and populate it believe to be true. e.g. Magic is Fluff, the Gods are fluff, the idea that dew from a Garden of Morr removes freckles and passes them on to the dead, that dwarfs can smell gold, that Necromancers must be nailed to a tree and burnt to death and the tree must then be uprooted and burned and that Beastmen will run away if you shout loudly at them and wave a torch, is all Fluff.
'Fluff' is very important because fluff is what brings the world to life. It's pure pizzazz and creativity and its unrestricted, uncontrolled, fun. But it can't be relied upon to define your world on its own. The reason being that 'fluff' is not necessarily true, nor is it necessarily consistent. In fact, one of the fun things about 'fluff' is that it doesn't have to be consistent. Different groups, cultures and organisations can have different versions of the 'fluff' and the argument and debate between them can even be the source of plots, adventures and even wars.
Is Sigmar a God? "Of course he is" roars the Sigmarite to say otherwise is heresy. But the Ulrican thinks not, they will argue that technically a mortal can never transcend to divinity. And so, the fluff creates debate and conflict, spawns theological treatise and spreads bigotry and dogma.

Metaphysics

'Metaphysics' are the rules that govern how the world really works. e.g. 'What goes up, always comes down.' (so gravity exists) The difference between 'Fluff' and 'Metaphysics' is that the world obeys the laws of metaphysics even if the people on it don't understand what those rules look like or how they work.
The academics can argue as long and hard as they like that the world is flat, but if the world is round, it remains stubbornly round.
So, its 'Metaphysics' that dictate what happens, but the explanation of why it happened is often 'Fluff'

Putting it all together

The vast majority of the text describing a world will be 'Fluff'. This is inevitable given that every 'Metaphysical' law is likely to spawn a number of in-game explanations. However, it helps if the 'Fluff' has at least a grain of truth or logic to it, if only from the perspective of those races, cultures or cults that promote it.

'Fluff doesn't have to be true, as long as its cool.'

'A Metaphysical rule has to be universally applicable and consistently applied.' 'There may be multiple fluff explanations of a single metaphysic rule.'

So, every race might have their own explanation for the existence of magic, but there can only be one true reason it exists in the world.