How to use namelist in vxml

How to use namelist in vxml:

<field name="dtmf">
            <option dtmf="1" value="1"/>
            <option dtmf="2" value="2"/>
            <option dtmf="2" value="3"/>
            <filled>
                <submit next="{url3}" namelist="action toneId dtmf" method="get"/>
            </filled>
</field>

The above shows the filled section in vxml file with namelist, what does it mean?

Thanks.

Please see my answer to your previous question; it gives details on how to use namelist.

As for why you should use namelist: namelist passes key/value pairs to the web service you access via <submit>. As with any other web service, the web service can use those key/value pairs to decide what response to send you.

As a concrete example, the web service might dynamically generate VoiceXML that contains annoucenements and options based on the values it receives.