expectedRevisionId

The write succeeds only if the named policy's current revision ID matches this value; a policy that does not exist yet matches only the sentinel "NO_POLICY" (create-only). On mismatch the operation fails with ConflictException. When omitted, the write is unconditional. Every attempt stores a newly generated revision ID, so retrying an unanswered request can conflict with the caller's own earlier attempt; read the policy back and compare it with the one you intended before treating a conflict as another writer's change.