From their site: You can pass other info in the outputs hashes, like labels, your own ids, or whatever. That extra info will be passed back to you in the respo

From their site: You can pass other info in the outputs hashes, like labels, your own ids, or whatever. That extra info will be passed back to you in the response JSON. So I assume it is a case of emailing them and asking for an exhaustive list of what can be passed to their api, the documentation definitely needs work.