The digest number really is just for us to use internally for tracking bounces that come back. It has no real significance otherwise and I wasn't sure whether to add it to the digest subject line or not. If it's confusing, perhaps I should just remove it.
In one of my Yahoo Groups I found the Digest numbers a handy way to check for email bouncing, greylisting, or blackholing. When I saw a gap in the digest numbers of a particularly busy group then I knew it was time to go pound on Verizon again (my ISP at the time). Not exactly a common or compelling use case though, hopefully that's not a problem I'll have again... ;-)
