DDE stands for "Dynamic Data Exchange". It is a communications protocol designed by Microsoft to enable all applications running in the Windows environment to send and receive data and to exchange instructions. It initiates a client/server relationship between two communicating applications.

Phoenix is fully equipped with DDE to communicate data bi-directionally between itself and MS-Windows applications such as Word and Excel.

You could use this facility to create & print invoice reports in Excel from data created by the Application and supplied by Phoenix, retrieved from the currently displayed Contact Record or an independent Query.