Before IoC was introduced in Magnolia, the way to replace the DefaultClassFactory with a custom one was by means of a Magnolia SystemProperty which info.magnolia.objectfactory.Classes would check via its getCurrentlyConfiguredClassName() method. Such property could be set by modules in the module descriptor e.g.

<properties>
    <property>
      <name>info.magnolia.objectfactory.ClassFactory</name>      
      <value>my.module.MyClassFactory</value>
    </property>
</properties>

Now, since we have IoC support in Magnolia, this way of configuring components has become legacy (see http://wiki.magnolia-cms.com/display/DEV/Components+in+Magnolia#ComponentsinMagnolia-Legacy), yet Classes keeps on using it to instantiate the desired ClassFactory implementation. Besides that, info.magnolia.cms.core.SystemProperty has been deprecated too in the meantime.
All the above premise to say that we should use IoC in this case too and no longer rely on properties.