private List<ApplicationContainer> createNodesFromContentServiceReference(ApplicationContainerCluster cluster, Element nodesElement, ConfigModelContext context) {
        NodesSpecification nodeSpecification;
        try {
            nodeSpecification = NodesSpecification.from(new ModelElement(nodesElement), context);
        } catch (IllegalArgumentException e) {
            throw new IllegalArgumentException(cluster + " contains an invalid reference", e);
        }
        String referenceId = nodesElement.getAttribute("of");
        cluster.setHostClusterId(referenceId);

        Map<HostResource, ClusterMembership> hosts = 
                StorageGroup.provisionHosts(nodeSpecification,
                                            referenceId, 
                                            cluster.getRoot().hostSystem(),
                                            context);
        return createNodesFromHosts(hosts, cluster, context.getDeployState());
    }
Allocate a container cluster without a nodes tag
config-model/src/main/java/com/yahoo/vespa/model/container/xml/ContainerModelBuilder.java