A cross-domain policy file is an XML document that grants a web client permission to handle data across multiple domains. When a client hosts content from a particular source domain and that content makes requests directed towards a domain other than its own, the remote domain would need to host a cross-domain policy file that grants access to the source domain, allowing the client to continue with the transaction. Policy files grant read-access to data, permit a client to include custom headers in cross-domain requests, and are also used with sockets to grant permissions for socket-based connections.

For more information on working with crossdomain.xml, refer to http://www.adobe.com/devnet/articles/crossdomain_policy_file_spec.html.