I don't know. Do you? Maybe @jmandel does?

Honesty though I never would have known that was even a thing unless you'd mentioned it. So I would have to do have researched it myself and probably not found a definitive answer because of the current state (read: confusing state) of medical record spec documentation ;)

I think it's better to not make assumptions on what certain systems would/wouldn't already know. If we purposely except this code/code system/code system name, how will devs know we haven't purposely excepted other fields as well? I guess we could explicitly document all the assumptions made, though that doesn't seem terribly scalable or efficient compared to just passing along the fields in the response.