This message will be variable in length, based on the amount of binary data. The length should vary between 1 and 5 slots.

Parameter	Number of bits	Description
Message ID	6	Identifier for Message 8; always 8
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; default = 0; 3 = do not repeat any more
Source ID	30	MMSI number of source station
Spare	2	Not used. Should be set to zero. Reserved for future use
Binary data	Maximum 968	Application identifier	16 bits	Should be as described in § 2.1, Annex 5
		Application data	Maximum 952 bits	Application specific data
Maximum number of bits	Maximum 1 008	Occupies up to 3 slots, or up to 5 slots when able to use FATDMA reservations. For Class B “SO” mobile AIS stations the length of the message should not exceed 3 slots For Class B “CS” mobile AIS stations should not transmit

 
Table 58 gives the number of binary data bytes (including application ID and application data), so that the whole message fits into a given number of slots. It is recommended that any application minimizes the use of slots by limiting the number of binary data bytes to the numbers given, if possible:

Number of slots	Maximum binary data bytes
1	12
2	40
3	68
4	96
5	121

These numbers also take into account bit stuffing.
Additional bit stuffing will be required for this message type. For details refer to transport layer, § 5.2.1, Annex 2.