The <topichead> element provides a title-only entry in a navigation map, as an alternative to the fully-linked title provided by the <topicref> element. The <topicgroup> element is for creating groups of <topicref> elements without affecting the hierarchy, as opposed to nested <topicref> elements within a <topicref>, which does imply a structural hierarchy. It is typically used outside a hierarchy to identify groups for linking without affecting the resulting toc/navigation output.