So, when I start to think of Heng and race, I immediately think of what I think most would consider to be the most basic of racial identifiers. Heng notes a lot of ways by which one might be identified with a race including region of origin, religious belief, etc., but I think the most common or basic of these would probably belong to visual identifier, by which I mean skin tone. Why do I think this? Well, it is the most readily available marker by which we can judge others or make a kind of power hierarchy. Also, it's a thing you can't hide. I think, primarily, of Heng's comments about Jews being persecuted for their belief in both modern and medieval. If the Jewish people were integrated into that society such that they were indistinguishable from the average, non-Jewish person. Putting aside the fact that no one should have to hide, regardless of ethical or religious identity, it would still be feasible for them to do so whereas a black person in a primarily white society would not be able to deny their blackness. They would not be able to escape persecution or judgement in the same way. To that end, I think that phenotypical characteristics are an easier thing on which to base something like race, but also the easiest thing to poke holes in because even the slightest amount of scientific rigor shows that they matter very little if at all.
Even so, we must consider what this does for power. When we consider one as another human or, even better, as a person, we recognize that, in their similarity to us they are owed every consideration that we should take for ourselves. But when we break humanity down to race, when we attach ourselves to something as trivial as the conditions of birth, we separate ourselves just enough to push the others out of humanity. By so doing, one makes an in built metric by which one can steal heaven from others for themselves. Race is useful to power because it creates, for the racist (using this here as one who fashions and uses race even though it is likely applicable in it's more mundane and valued sense), an entire army of people who one not only feels that they can control, but should by right of birth. It is the reduction of people to animals and objects which are to be used and traded.