A refined product is not defined by what it is made of but by whether it meets a specification. Those specifications are set by regulators and by standards bodies, they differ by region and by season, and they change. A stream that is saleable gasoline in one market is off-spec material in another.
Three families of specification do most of the work. Sulphur limits, which have ratcheted downward across road fuels, marine fuels and now increasingly everywhere. Volatility limits on gasoline, expressed as Reid vapour pressure, which are tightened for summer to limit evaporative emissions. And performance specifications such as octane for gasoline and cetane for diesel, which determine whether the fuel works in the engine it is sold for.
The seasonal gasoline switch is the clearest example of a specification creating market structure. Summer-grade gasoline has a lower permitted vapour pressure, which means less butane can be blended in, which reduces the volume of saleable gasoline made from the same crude. The changeover has a fixed date, inventories of winter-grade product have to be cleared before it, and the whole thing produces a reliable seasonal pattern in gasoline cracks that has nothing to do with driving demand.
The marine fuel sulphur reduction is the largest recent example of specification changing the crude market rather than only the product market. Cutting the permitted sulphur in bunker fuel repriced high-sulphur residue downward and low-sulphur material upward, which changed the relative value of sour and sweet crude, which changed quality differentials worldwide. A rule about ships repriced the crude complex.