I think subtitles and captions are the same foe YouTube-DL. However, some videos only have auto-generated captions, and YouTube-DL doesn't download these... by default. There is an option to force the download of both generated and even translated captions.
Which is out of a default quota of 10,000 units per day, and one can request an increase if your actually need more, (which presumably gets granted if YouTube agrees your use case is not an abuse of their API).