the xml you show is not required to configure the current version of Struts. It may be a remnant from an earlier version.
Currently, the ApplicationResources.properties file is the default name given to the file used to retrieve messages based on message keys. However, you can name this file anything you want. It's configured in the struts-config.xml file as follows:
<message-resources parameter="com.mycompany.resources.ApplicationResources"/>
This means that struts will look for ApplicationResources.properties in the directory myApplication/WEB-INF/classes/com/mycompany/resources
If you're using the internationalization feature of struts, you may also have other files such as ApplicationResources_de.properties, ApplicationResources_fr.properties, etc. for different languages.
The reason you don't see this file in any of the Struts examples is that they've configured their file name as MessageResources instead of ApplicationResources.
See
this link for further information on this file and how it relates to Struts internationalization.