This case-sensitive code system http://hl7.org/fhir/messageheader-response-request defines the following codes:
| Code | Display | Definition |
| always | Always | initiator expects a response for this message. |
| on-error | Error/reject conditions only | initiator expects a response only if in error. |
| never | Never | initiator does not expect a response. |
| on-success | Successful completion only | initiator expects a response only if successful. |