What's an API response that doesn't involve resources?
API calls that send a response that's not a resource per se are not uncommon depending on the domain. I've seen it in financial services, Telco, and the automotive domain to some extent.
Actions like the following are your clue that you might not be dealing with a "resource" response.
For example, you want to make a simple algorithmic calculation like how much tax someone should pay, or do a natural language translation (one language in request; another in response), or convert one currency to another. None involve resources returned from a database.
In these cases:
Use verbs not nouns
For example, an API to convert 100 euros to Chinese Yen
Make it clear in your API documentation that these are different.
Simply separate out a section of documentation that makes it clear that you use verbs in cases like this because these calls are not resource related.
Next time: what about searching?