The layer starts EMPTY and inherits everything above it — a layer only ever adds. An optional body {agent_ids} pins those agents to the new zone by hand in the same transaction (at most 5 000; an id that is not an agent of this organization answers 404 AGENT_NOT_FOUND). The bare call, with no body, stays valid. 409 ZONE_EXISTS when the unit already carries one.
Optional. Makes the request safe to retry: the same key and the same request, within 24 hours, returns the stored response with Idempotent-Replayed: true and runs once.
Agents pinned to the new zone by hand in the same transaction (at most 5 000; an id that is not an agent of this organization answers 404 AGENT_NOT_FOUND).