Subject line vs. V number

["Daegrigg (Don)" <daegrigg@domain.elided>]

I notice that some replies to the Digest reference a volume and message number in the subject line.  Where does this come from.  I don't see it in messages I received.  'Not a big deal except it slows down scanning of new messages since you cannot tell what the message is in reply to from just the volume number, etc. 

Just curious. 
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