Aug 17, 2005 - Understanding crossdomain.xml
The Flash Player follows a sandbox security. In certain cases you'll need to provide a crossdomain.xml file for your Flex Applications to work

A Collection of crossdomain.xml knowledge:

- Why crossdomain.xml is a good thing by Mrtijn de Visser
"...every SWF file you view runs locally on your machine. This means that a SWF would have HTTP access to all machines behind the company firewall. To prevent this, every server other than the one the SWF is loaded from, needs to have a crossdomain.xml file in its root, listing all domains that have access to that particular server..."
http://www.martijndevisser.com/archives/000056.php

- Cross Domain Policy Files by Colin Moock
"This technote gives a brief overview of the issue, and outlines the most common way.."
http://moock.org/asdg/technotes/crossDomainPolicyFiles/

- External Data not accessible outside a Flash movie's domain - Macromedia Tech Note
"...with this change a sub-domain can no longer read data from a parent domain and vice versa..."
http://www.macromedia.com/cfusion/knowledgebase/index.cfm?id=tn_14213

- Using Policy Files for Macromedia Flex - Macromedia Flex Live Docs
"To make data and assets in runtime shared libraries to SWF files in different domains, use a cross-domain policy file..."
http://livedocs.macromedia.com/flex/15/flex_docs_en/wwhelp/wwhimpl/common/html/wwhelp.htm?context=Flex_Documentation&file=00000894.htm