Used for tables.

<informaltable>
 <tgroup cols="2">
  <colspec colwidth="30*"/>
  <colspec colwidth="50*"/>
  <tbody>
   <row>
    <entry>
     <emphasis role="bold">Operating System</emphasis>
    </entry>
    <entry>
     <emphasis role="bold">File-size Limit</emphasis>
    </entry>
   </row>
   <row>
    <entry>
     Linux-Intel 32-bit
    </entry>
    <entry>
     2GB, much more when using LFS
    </entry>
   </row>
  </tbody>
 </tgroup>
</informaltable>

See also tgroup, tbody, row, and entry.

http://docbook.org/tdg/en/html/informaltable.html