One element references another when an attribute of the referencing element has the same value as an attribute of the referenced element. For example, the application-ref element references an application or module that is deployed to its parent server element. The application-ref element’s ref attribute has the same value as the name attribute of a lifecycle-module, j2ee-application, ejb-module, web-module, connector-module, or appclient-module element.
The referencing application-ref element might look like this:
<application-ref ref="MyServlet"/>
The referenced web-module element might look like this:
<web-module name="MyServlet" location="myservletdir"/>