The map group domain elements define, group, or reference content. Many of the map group elements are convenience elements, which means that they simply make it easier for an author to make use of existing functions.
For example, the <topichead> element allows a map to specify a heading
without allowing a reference to a topic. While a <topicref>
element might accomplish the same thing by creating a
title and leaving off the @href attribute, the
<topichead> element simply makes the intent clearer and
prevents the accidental inclusion of an @href attribute.