The DDEML notifies an application about DDE activity that affects the application by sending transactions to the application's DDE callback function. A DDE transaction is similar to a message — it is a named constant accompanied by other parameters that contain additional information about the transaction.
The DDEML passes a transaction to an application-defined DDE callback function that carries out an action appropriate to the type of transaction. For example, when a client application attempts to establish a conversation with a server application, the client calls the DdeConnect function. This function causes the DDEML to send an XTYP_CONNECT transaction to the server's DDE callback function. The callback function can allow the conversation by returning TRUE to the DDEML, or it can deny the conversation by returning FALSE. For a detailed discussion of transactions, see Transaction Management.