Manual

3.4. metal:fill-slot

This attribute occurs only under metal:use-macro context.

This explicitly tells PHPTAL to replace a defined slot with the content provided under the metal:fill-slot attribute.

<span tal:condition="logged" metal:fill-slot="news_place">
  <h2>user menu</h2>
  <ul>
    <li><a href="/user/action/inbox">inbox</a></li>
    <li><a href="/user/action/new">new mail</a></li>
    <li><a href="/user/action/disconnect">disconnect</a></li>
  </ul>
</span>

Slots give the opportunity to define really customizable and reusable page templates with a simple push technology.