Your question is a little vague, but I'll take a whack at it... The concept applies to clients that aren't browsers where the client needs to know about your
java classes.
I did a search for "application client deployment descriptor" on bea docs website
http://search.bea.com/weblogic/go60/ Next URL describes the "WebLogic Run-time Client Application Deployment Descriptor" in great detail:
ClientMain.runtime.xml
http://e-docs.bea.com/wls/docs60/////programming/app_xml.html#1032618 Next URL is about "Packaging and Deploying Client Applications"
and it says "Although not required for WebLogic Server applications,
J2EE includes a standard for deploying client applications. A J2EE Client application module is packaged in a .jar file. The .jar file contains the Java classes that execute in the client JVM and deployment descriptors that describe Enterprise JavaBeans and other WebLogic resources used by the client.
A standard deployment descriptor from Sun is used for J2EE clients and a supplemental deployment descriptor contains additional WebLogic-specific deployment information. "
http://e-docs.bea.com/wls/docs60/programming/packaging.html#1045386