The macro define-dde-client defines a mapping from the symbol name to the DDE service name with which to establish a conversation, and the conversation class to use for this conversation. The argument service is a string which names the DDE service. It defaults to the print-name of name. The argument class is a subclass of dde-client-conversation which is used for all conversations with this service. It defaults to dde-client-conversation. Specifying a subclass allows various aspects of the behavior of the conversation to be specialized.
Note that it is generally not necessary to register client services unless a specialized conversation type is required. However, it is sometimes convenient to register a client service in order to allow the service name to be changed in the future.
If the macro executes successfully, the name of the DDE service is returned.
LispWorks User Guide and Reference Manual - 20 Sep 2017