The Deployment Descriptor is an XML file that configures an app with certain settings when the app is deployed or restarted. For example, the Deployment Descriptor can configure the app with a certain context root, such as /beta.
The Deployment Descriptor XML file can be viewed in the WebSphere admin console or on the server that is running WebSphere. To view the Deployment Descriptor in the WebSphere admin console, you will navigate to Applications > All applications > your application > View Deployment Descriptor.
In this example, when Delta.ear is deployed to WebSphere, the display name of the EAR will be devWASDelta001, and the EAR contains a WAR (beta.war), where the context root of the WAR will be /beta, and a security role called Authenitcated will be created.
In this example, the Deployment Descriptor is almost identical to the application.xml file in the EAR. The only difference is that the Deployment Descriptor has one additional line - <initialize-in-order> False</initialize-in-order>.
Different files are read by WebSphere when an app is deployed or restarted based on the type of file that has been deployed.