While currently playing Prey I noticed that somebody marked Closed Captions for this game as true. And while it does offer subtitles for all dialogues, it only partially offers them for sound effects. For example, the sound of some of the robotic devices gets described if you are near them, but things like shooting, breaking glass, explosions or alien noises do not.
The editing guide mentions "Text accompanied with all audio in-game." so in this case it should probable be false then, right?
However, I've checked a couple of pages which makes me unsure about the setting in general. Dragon Age Origins has it enabled and while it is heavily text based, things like sounds (or combat chatter IIRC) has no subtitles. Same goes with other classical RPGs.
Now looking at some Bluray movies with SDH subtitles, not every noise necessarily gets described - at least not in dramatic/action scenes.
So I think we need some better clarification as to when CC should be true, false or N/A. Especially with the different genres in mind (e.g. adventure games). I know this is generally a rare feature but it seems even the Steam store often lists it for games which only feature normal subtitles.