for a-subha (non-beauty, foul perception), Ajahn Mun points out the skin (from 31asb body parts) is especially important

Ajahn Mun: "When we get infatuated with the human body, the skin is what we are infatuated with. When we conceive of the body as being beautiful (subha) and attractive, and develop love, desire, and longing for it, it’s because of what we conceive of the skin. When we see a body, we suppose it to have a complexion—fair, ruddy, dark, etc.—because of what we conceive the color of the skin to be. If the body didn’t have skin, who would conceive it to be beautiful or attractive? Who would love it, like it, or desire it? We’d regard it with nothing but hatred, loathing, and disgust. If it weren’t wrapped in skin, the flesh, tendons, and other parts of the body wouldn’t hold together and couldn’t be used to accomplish anything at all—which is why we say the skin is especially important. The fact that we can keep on living is because of the skin. The fact that we get deluded into seeing the body as beautiful and attractive is because it has skin. This is why preceptors teach only as far as the skin.

If we set our minds on considering the skin until we see it as disgusting and gain a vision of its unloveliness appearing unmistakably to the heart, we are bound to see the inherent truths of inconstancy, stress, and not-selfness. This will cure our delusions of beauty and attractiveness that are fixated on the skin. We will no longer focus any conceivings on it or find it appealing or desirable, for we have seen it for what it is." - From https://www.dhammatalks.org/books/HeartReleased/Section0005.html