This code system http://hl7.org/fhir/response-code defines the following codes:

CodeDisplayDefinition
okOKThe message was accepted and processed without error.
transient-errorTransient ErrorSome internal unexpected error occurred - wait and try again. Note - this is usually used for things like database unavailable, which may be expected to resolve, though human intervention may be required.
fatal-errorFatal ErrorThe message was rejected because of a problem with the content. There is no point in re-sending without change. The response narrative SHALL describe the issue.