Where the shape comes from
A boundary line bent into three sides of a box, open at the bottom — the far frontier as early writers pictured it, a wall around the settled world with nothing marked beyond. It frames 再, a stand holding a second load ready to be added once more, and gathers 冈, a mountain ridge seen from a distance, and 册, the bundle of bound bamboo strips that made an early book or volume.
汉字 filed under 冂
No published character on HSK Guide is filed under this radical yet. 汉字库.