The note after “No” says the withholding rate is 20% unless tax law stipulates otherwise. That’s the standard withholding rate for 執行業務 (9A) aka income from professional practice, which I would say is the most likely category for any freelance income other than 稿費 (9B).
It should be easy to declare it as 薪資 (50) if the two parties are clearly in an employment relationship.