Dynamic Data Exchange is a mechanism for interprocess communication. DDE is based on the messaging system built into Windows. Two Windows programs carry on a DDE "conversation" by posting messages to each other. These two programs are known as the "client" and the "server." A DDE server is the program that has access to data or features that may be useful to other Windows programs. A DDE client is the program that obtains this data from the server.
DPlot can act as both a DDE client and a server.