The group tag is used only within the body of a multiple tag to provide additional formatting control between subsets of a multirow data source. The tag takes a column name from the enclosing multiple tag as its only attribute. It repeats a template section as long as the value of the column does not change from row to row.
The group tag alsos sets two additional values in your multirow:
<table>
<multiple name="shirts">
<!-- Start a new row if the style changes -->
<tr>
<td>
@shirts.style@
</td>
<td>
<!-- List colors for the same style in a single cell -->
<group column="style">
@shirts.color@
<!-- @shirts.groupnum@ will be the number of the color within the style -->
<if @shirts.groupnum_last_p@ false>, </if>
<else>, or </if>
</group>
<!-- End the row if the style is going to change on the next row
</td>
</tr>
</multiple>
</table>
Group tags may be nested to perform even more complex formatting.