Where the shape comes from
The Shuowen reads it as a barb — a shaft with the point turned back so that what it catches cannot slip off. Nobody says the word; it survives as the index heading for characters whose spine is a hooked vertical, and it takes in two of the commonest graphs in the language: 了 and 事. 予 and 争 sit here too.
汉字 filed under 亅
No published character on HSK Guide is filed under this radical yet. 汉字库.