I asked the same thing internally, and Travis replied like this:

These are the "rating descriptors" or "rating reasons". Some countries mandate these as part of the rating laws around content. Some countries have them on only one of either the movie or TV show ratings (the US has them on TV for example), and some have them for both (Brazil has them for both movie and TV). And some countries don't have any. You'll notice the descriptors field only appears on the countries we have added support for, so this option will not appear for every country.

Edit: I assume something will be added to the Contribution Bible soon.

That makes sense, I was wondering if it was for that type of thing ("violence, language"). That's cool!

@travisbell Please use the same filtering for the Language field, and only add the support for India, Switzerland and Canada. Many users don't understand the meaning of this field in the first place, so this would greatly reduce the unnecessary use of this field.

I've been using that field (in absence of any specificity) to record the data of companies which released and distributed the film, whether it was edited or subtitled or dubbed (in the case of foreign releases) and so on. What other fields could that data go in? None seem to be available under those specific headings. As with genres, some things missing...

@Rich Wannen said:

I've been using that field (in absence of any specificity) to record the data of companies which released and distributed the film, whether it was edited or subtitled or dubbed (in the case of foreign releases) and so on. What other fields could that data go in? None seem to be available under those specific headings. As with genres, some things missing...

Note that data for dubbed/alternative versions is not supported yet, even though we of course allow release dates for movies only released as a dub (common for children's movies) in a country.