Message 13 should be used as an acknowledgement of up to four Message 12 messages received (see § 5.3.1, Annex 2) and should be transmitted on the channel, where the addressed message to be acknowledged was received.
These acknowledgements should be applicable only to the VHF data link (see § 5.3.1, Annex 2). Other means must be employed for acknowledging applications.

Parameter	Number of bits	Description
Message ID	6	Identifier for Messages 7 or 13 7 = binary acknowledge 13 = safety related acknowledge
Repeat indicator	2	Used by the repeater to indicate how many times a message has been repeated. See § 4.6.1, Annex 2; 0-3; 0 = default; 3 = do not repeat any more
Source ID	30	MMSI number of source of this acknowledge (ACK)
Spare	2	Not used. Should be set to zero. Reserved for future use
Destination ID1	30	MMSI number of first destination of this ACK
Sequence number for ID1	2	Sequence number of message to be acknowledged; 0-3
Destination ID2	30	MMSI number of second destination of this ACK; should be omitted if no destination ID2
Sequence number for ID2	2	Sequence number of message to be acknowledged; 0-3; should be omitted if no destination ID2
Destination ID3	30	MMSI number of third destination of this ACK; should be omitted if no destination ID3
Sequence number for ID3	2	Sequence number of message to be acknowledged; 0-3; should be omitted if no destination ID3
Destination ID4	30	MMSI number of fourth destination of this ACK; should be omitted if no destination ID4
Sequence number for ID4	2	Sequence number of message to be acknowledged; 0-3. Should be omitted if there is no destination ID4
Number of bits	72-168