Captions like “[ upbeat music ]” or “[ computer beeps ]” signal to deaf listeners information about the program that they cannot sense. There is no need for those when adding subtitles to a foreign-language program made for a hearing audience.

Similarly, subtitles are necessary for on-screen foreign language text (e.g., signs or documents), but not when the text is in the language that a deaf viewer understands. In other words, you wouldn’t have an English subtitle for an on-screen sign in English, but you would for a sign in Chinese.

As the BBC produces content exclusively (or nearly exclusively) in English, these guidelines are surely directed towards captioning for hearing-impaired audiences.