Which anatomical terms need to be used in relation to another body part, like saying 'the ears are lateral to the eyes', and which need to be used absolutely, like 'the skin is superficial', and which can be used in both ways?

Terms of relationship are not absolute. They are called terms of relationship because they describe how parts of the body are located in space relative to other body parts. Median may be considered an absolute term, as it means in the midline. Other terms, however, describe how a body part lies in relation to another part. Perhaps the soles of the feet could be described as absolutely inferior to everything else in the body, but they are still inferior relative to the rest of the body. Thus, these terms are not used as absolutes.