The index of the atlas's own group level pages, the narrower listings minted under a subcategory so a long list of entries is broken into smaller browsable slices rather than handed to a reader as one flat page. It exists so the group layer itself is browsable, not because a group is a subject of study in its own right, and it lists every group the atlas currently carries. It excludes every entity that is not itself a group node, and excludes the individual entries a group lists, which hang their own facts off their own substantive type.