Where the shape comes from
The earliest form is disputed — a ladle, a blade and a root hanging from a stem have all been argued for — but the sense that came down is a branch of a family line, the 氏 of 姓氏 and of a woman’s maiden name. As a component it works for sound: directly in 纸, and through the closely related 氐 in 低 and 底.
汉字 filed under 氏
No published character on HSK Guide is filed under this radical yet. 汉字库.