realm_alias_taken· reason_code· applies to state_resolution, service_callAn ak.realm.alias declaration requested a canonical alias already held by a different Realm in the same issuing authority's realm-alias namespace. The alias registrar MUST reject the later claim rather than re-pointing the alias; releasing an alias requires the holding Realm to publish an ak.realm.alias tombstone first. Handle namespace occupancy is NOT a collision (the two namespaces are disjoint). See zh/discovery/object-addressing.md §3.3.