The two parties to a DDE conversation (connection or link) are known as the server and client. The server makes itself available by registering itself with Windows. The client initiates the conversation by sending a message to the server.
The DDE protocol is asymmetric: the client initiates the conversation, sends messages and instructions to the server, and terminates the conversation. The server cannot by itself initiate a conversation, or send messages except in response to a client request.