notes, comment, annotate
註 centers on adding explanatory text to a document: writing notes, comments, or annotations that clarify or supplement the main content.
註 combines 言 (words, speech) with 主, which likely contributes the sound チュウ and may also suggest the idea of 'main' or 'master' text that is being annotated.
Words (言) beside the main text (主) are notes and comments added to explain it.
For チュウ, imagine you are writing notes in the middle (中, ちゅう) of a page: 中 -> チュウ, and your annotations fill the margins.
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