On February 1, 2018, U.S. Customs and Border Protection (CBP) announced in Cargo Systems Messaging Service (CSMS) #18-000107 that they added shipment hold and release messages for export shipments to the Automated Export System Trade Interface Requirements (AESTIR) Appendix A.
The messages are currently inactive and a follow-up message will announce when they will be active.
The CSMS can be found here:
The AESTIR Appendix A can be found here:
https://www.cbp.gov/document/guidance/aestir-appendix-commodity-filing-response-messages