This tag is specifically meant for illustrations that excellently depict the female buttocks (the ass). PLEASE DO NOT APPLY IT TO YOUR OWN ILLUSTRATIONS. If other viewers think the image warrants this tag, they will compliment you by adding it.
This tag is reserved for illustrations featuring buttocks that are not only prominent, but also exhibit an excellent sense of weight, elasticity, and texture among other desirable qualities. Additionally, the woman's body and face should be appropriately pleasing as well. Personal preference will certainly factor into whether each individual feels whether this tag is deserved, but please respect the wishes of others if they have added this tag.
While a majority of the illustrations with this tag are likely to be rated R-18, it is applicable to any image you feel is among the "best" in this category.
This tag is related to other tags with "極上の___" ("best of ___") and/or "___神様" (Heavenly ___) in the tag, each related to different subject matter the members of Pixiv feel is an excellent example there-of.