The subgraph's name in the DOT source, such as cluster_build.
What the cluster is called: its drawn label, or, when it draws none, its
name without the cluster prefix Graphviz requires.
The labels of the clusters it is drawn inside, outermost first. Empty for a cluster at the top level.
The cluster's group.
One cluster Graphviz drew: a
<g class="cluster">, asubgraph cluster_*.