DDE_ACK

Invoked in a client or server when a positive acknowledgement has been received from the partner.

DDE_ERROR

Invoked in a client or server whenever the DDE DLL detects an error.

DDE_NACK

Invoked in a client or server when a negative acknowledgement has been received from the partner.

DDE_TERMINATE

Invoked in a client or server when a partner terminates a conversation.