How does the captcha valid. in forms work?

[321los]

Hello,
I know how to put in in the form and make it work, but what I do not understand is how it works, especially whats in the sitemap

you put s.th like that in your form:
<fd:captcha id="captcha" required="true" length="5">
       <fd:datatype base="string"/>
            <fd:validation>
                <fd:captcha/>
            </fd:validation>
 </fd:captcha>

sitemap:
<map:match pattern="captcha-*.jpg">
      <map:generate type="jx" src="">        <map:parameter name="capt" value="{session-attr:captcha-{1}}"/>
      </map:generate>
      <map:serialize type="svg2jpeg"/>
  </map:match>

does it mean this: <fd:captcha id="captcha" required="true" length="5"> calls that: <map:match pattern="captcha-*.jpg">
and what about this entry: value="{session-attr:captcha-{1}}" ??

A description of the whole process would be great....
Thank you
Marco