Combination of rich:dataTable and rich:subTable allows to show "master-details" relationship.
For example, the table from the richfaces-demo (http://livedemo.exadel.com/richfaces-demo/richfaces/dataTable.jsf) integrate information about the cities that employee visited (master table) and the details of expenses for each city (master table).
From the table layout perspective, subTable instead of dataTable has no own tag, but incorporated into the parent dataTable. It means that both dataTable and subTable have common aligned columns.
The cool feature of rich:dataTable is ability to join columns and rows (colspan, rowspan) if needs.
See also the incomplete yet article about rich:dataTable: