Accessing services on web sites other than the web start clients own domain?

I am working on a web start client in java. I need to connect to other servers than from were the client is downloaded from.开发者_Go百科 I thought I need to sign my jars but this (http://download.oracle.com/javase/tutorial/deployment/deploymentInDepth/signing.html) document describes that:

Accessing services on web sites other than the RIA's own domain – In the past, RIAs could only communicate with the server from which they were launched. The Java Plug-in software and the Java Web Start software now support cross-domain policy files. If a third-party site has set up cross-domain policies and allows access from other domains, your unsigned RIAs can invoke services from this site.

What does this mean and how do I set this up and what are cross-domain policy files?

Thanks.

Here is a blog post describing using crossdomain.xml with Java.

Basically, you need to put a crossdomain.xml file on the server of the web service you want to call, which allows the domain that your app will be launched from.