The MFD transaction is the delayed application acknowledgment. It can be used to return "deferred" application-level acknowledgment statuses at the MFE level, without reference to the original MFN message. It is defined as follows:
MFD Master File Delayed Acknowledgment Chapter
MSH Message Header 2
MFI Master File Identification 8
{ [MFA] } Master file ACK segment 8
ACK General Acknowledgment Chapter
MSH Message Header 2
MSA Acknowledgment 2
[ ERR ] Error 2