A collection type is one of a group of attributes used to create relationships among "sibling" topics that share a common parent.
To specify a collection-type attribute for a group of topics that do not have a common parent (for example, topics listed in the same cell of a relationship table, use the topic group container element.
Example
<reltable> <relrow> <relcell> <topicgroup collection-type="family"> <topicref href="../release_current/sysreqs.dita"/> <topicref href="../installing/installing_overview.dita"/> </topicgroup> </relcell>