The DDEML notifies an application of DDE activity that affects the application by sending transactions to the application's DDE callback function. A 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, which carries out the appropriate action depending on the type of the transaction. For example, when a client application attempts to establish a conversation with a server application, the client calls the DdeConnect function. This causes the DDEML to send an XTYP_CONNECT transaction to the server's DDEcallback 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 Section 5.8, “Transaction Management.”