A helper component, subTable can be used to group row data inside a table.
A basic definition of a table that contains subTable is given here:
<p:dataTable value="#{dataTableBean.boxers}" var="boxer">
<f:facet name="header">
Boxers
</f:facet>
<p:columnGroup type="header">
<p:row>
<p:column rowspan="2" headerText="Boxer" />
<p:column colspan="2" headerText="Stats" />
</p:row>
<p:row>
<p:column headerText="Wins" />
<p:column headerText="Losses" />
</p:row>
</p:columnGroup>
<p:subTable var="stats" value="#{boxer.stats}">
<f:facet name="header">
<h:outputText value="#{boxer.name}" />
</f:facet>
<p:column>
<h:outputText value="#{stats.match}" />
</p:column>
<p:column>
<h:outputText value="#{stats.win}" />
</p:column>
...