In this example, ,terminology and ,comment form a subgroup in the Hierarchy group.
They make up the Hierarchy.Demonstrator subgroup.
An alternative formulation describes ,terminology and ,comment as subpages of the Hierarchy.Demonstrator page. In the subpage formulation, there is no requirement for a designated “group page”. The two formulations are equivalent; use whichever makes more sense in a given situation.
In this demonstrator implementation, a subpage has only one parent and a subpage has siblings, but no children.
If a local/Hierarchy.Demonstrator.php script exists, it loads for the subpages too. In this way, PmWiki already supports Group, Subgroup and Page customisations.