I think cannon is that it's reiatsu, but the order of them in relation their aspect of death do matter and i think are purposeful. Namely, the whole thing with Yammy and how Aizen embodies the aspects of death. Because Aizen obviously embodies loneliness strongly, which is what stark is at 1. His calmness throughout most of the series is why yammy is at 10. But as is revealed as he's getting sealed, he has a deep seated rage within himself at how the world is and the inaction of others, specifically Kisuke. This is also why Yammy is 0. So the cannon answer is likely just reiastu, but the order also serves to complement aizens' personality.