what is mean by j2ee deployment descriptore and runtime deployment descriptor?

please help me this is interview question?
what is the use of two different type of deployment descriptor?
1 J2EE deployment descriptor.
2 runtime deployment descriptor

I dont know what is run-time deployment descriptor. J2EE deployment descriptor is assocaiated with J2EE application and configures the deployment component. For example application.xml is assocaiated with ear application.

More here
http://java.sun.com/blueprints/guidelines/designing_enterprise_applications_2e/deployment/deployment5.html

There are two types of deployment descriptors: J2EE and runtime.

A J2EE deployment descriptor
is defined by a J2EE specification and can be used to configure deployment settings on any J2EE-compliant implementation.

A runtime deployment descriptor
is used to configure J2EE implementation-specific parameters. For example, the Sun Java System Application Server Platform Edition 8 runtime deployment descriptor contains information such as the context root of a Web application, the mapping of portable names of an application's resources to the server's resources, and Application Server implementation-specific parameters, such as caching directives.

The Application Server runtime deployment descriptors are named sun-moduleType.xml and are located in the same directory as the J2EE deployment descriptor.

Reference Website
http://java.sun.com/j2ee/1.4/docs/tutorial/doc/Overview5.html

So A runtime deployment descriptor is a vendor specific DD.