This attribute declares a macro. Think of macros as library of small templates which can be reused in any other template.

<div metal:define-macro="main_menu">
  <ul>
    <li><a href="/">home</a></li>
    <li><a href="/products">products</a></li>
    <li><a href="/contact">contact</a></li>
  </ul>

  <div>
    Last modified:
    <span tal:content="mdate">page modification date</span>
  </div>
</div>

Macros inherit from the caller's dictionary. In the above example, the variable 'mdate' depends on the template that calls the macro.

This attribute calls a macro and includes its result in the current template.

<span
  tal:comment="main_menu template requires 'mdate' variable"
  tal:define="mdate page/last_modified"
  metal:use-macro="main_menu"
/>

You can refer to external macros defined in other templates by specifying the template source file.

<span metal:use-macro="site_macros.html/main_menu"/>

It is interesting to note that you can also use the PHPTAL inline replacement feature inside the use-macro attribute value:

<span metal:use-macro="${design}/site_macros.html/main_menu"/>

This attribute must appear under a metal:define-macro tag.

Slots can be replaced by caller template with some custom dynamically-generated XML/XHTML.

Slots can be thought of like reverse includes, a macro can be an entire page and slots customize this page depending on the URL. For instance, a slot may contain the latest news in the home page or user actions when the member is logged in.

<span metal:define-slot="news_place">
  <table>
    <tr tal:repeat="item php:latestNews()">
      <td tal:content="item/value">news description</td>
    </tr>
  </table>
</span>

The above example defines a place called 'news_place' which can be overwritten by caller templates. See next section for the continuation of this example.

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.