档
dǎng , dàng · HSK 6
grade
📁 档 = 木 wood + 當 block; wooden slat that blocks/files things into slots
The wood radical 木 plus 當 (block, take charge) gives the senses of a filing slat or gear-stop. Sounds like dǎng/dàng — same root as 挡 (to block). Hence: gear, files, grade-slot.
档 — wood radical 木 + 当 (dāng) phonetic
档 is a semantic-phonetic compound: 木 ('wood') signals the meaning, since the original referent was a wooden shelf or rack for storing files. 当 supplies the sound. From 'wooden file rack' the meaning extended to 'archive,' 'file,' and 'grade/category.'