DDE Client. Applications that request information from other DDE applications are said to be DDE client.
DDE Server. Applications that supply information via DDE to requesting applications are known as DDE server.
DDE does not define the format for the information being passed in a conversation. In most cases, the DDE client requests data in the format it prefers. If the server does not return that data, the client then requests its second choice. This process continues until the DDE server responds with data in the requested format or the client stops requesting the information.
Microsoft Access can act as either a DDE client or a DDE server.
DDE conversations are one way. The client application is always the one that issues the transactions. If the server needs to issue a transaction to the client, a new conversation needs to be started. The server is now a client.