Our understanding is that sync or async should be decided by the requesting side, and the responding system must follow it. Not the other around. (see FHIR Operations page: https://www.hl7.org/fhir/messageheader-operation-process-message.html)

Existing Wording:

Specialty Rx Query Responses are expected to be systematically produced using data in the patient’s electronic health record. They may be returned synchronously or asynchronously in near real-time. The Requesting System cannot influence which behavior occurs; when a message is received, the Data Source System determines whether it will respond synchronously or asynchronously.

(Comment 21 - imported by: Jean Duteau)