「Buttocks cracks spread and arrayed with spread legs」or「genitals with the registry of concubines」of the harem, meaning FLAT, since they would have very little to do almost all days and nights, hence sitting upon one's ass all the time (with those no-longer-integrated feet, remember?) would naturally FLATTEN ONE SIDE (…perhaps more comedic relief of the Chinese sages here?).
　「Buttocks cracks spread and arrayed with spread legs」would be most likely that counting, inspecting, and recording data from such a scene would be best accomplished by requesting everyone to lie FLAT upon the ground or floor, perhaps for some sort of an inspection of the harem, harem candidates, or routinely performed with the shaman's service providers for quarantining when infected.
　KNOTWEED grows relatively FLAT upon the surface of soil from a single deep taproot sending branches in various directions, resembling in many ways the shape of an anal sphincter with many sets of spread legs bound to the soil only ON ONE SIDE, ON ONE END…imagine that.
　A written record or「official harem journal of spreads buttocks crack」suggests laying a large and rather heavy set of bound pages open, FLAT upon a table, with the slightly curved pages on each side of the binding seam appearing as does a「spread buttocks crack.」BOUND ON ONE SIDE is the same journal in a closed position. A「register of this buttocks crack」is a HORIZONTAL TABLET with writing as a nameplate tablet horizontally placed on one side of a home's doorway, recalling that at first, folks generally lived in pit holes underground, so that an identifying ADDRESS PLATE would necessarily have been FLAT HORIZONTALLY on one side of the flat, horizontal door.

Modern definitions (that generally disregard history) …excluding politically incorrect concepts and other meanings deemed offensive today; may list only pigeonholed definitions, euphemisms, or meaningless mnemonics)
Chinese: Polygonum aviculare (knot weed); small boat; flat; (old form of character 匾, horizontal tablet with inscription)
Japanese: flat, even, level, simple, plain
Unihan extended: flat; tablet, signboard
EDRDG: level; small