The ACL now writes the domain registrationId as the zaak's identificatie on POST /zaken, and exposes GET-through POST /zaken/reference to read a zaak's identificatie back. Only the ACL touches ZGW (§8.1); the reference is the single value shown on both the self-service confirmation and the openbaar register. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>