[Larry Tonnade]

Now I think I have found the appropriate explanation here :

The "message sequential identifier" is a number from 0 to 9 that is sequentially assigned as needed. This identifier is incremented for each new multi-sentence message. The count resets to 0, after 9 is used. For radio broadcast messages requiring multiple sentences, each sentence of the message contains the same sequential identification number. The purpose of this number is to link the separate sentences containing portions of the same radio message. This allows for the possibility that other sentences might be interleaved with the message sentences that contain the complete message contents. This number also links an ABK-sentence acknowledgement to the appropriate BBM-sentence.

I havent seen anything as clear elsewhere.
My source:http://www.pamguard.org/devDocs/AIS/AISDataUnit.html