We don’t actually accept subjects (or keywords) in the metadata that members supply while registering their DOIs.
The subjects that you see in the API were added basically as an experiment, based on the Scopus ASJC subject codes. They’re one of the few things in the API metadata records that’s added by Crossref directly, rather than coming from the publisher/member.
Those subjects are of limited use, because they only apply to journals which are indexed by Scopus. They tend to cause a lot of confusion as well, because they only pertain to the subject of the journal as a whole, which often doesn’t align well with the subject of a given article.
We are planning to remove them in a future update to the REST API, though I don’t have a timeframe for that change.
So, that’s all to say, you don’t need to worry about which subjects to put in your metadata deposits, because our schema doesn’t allow for that.