I assumed the SeqNo was assigned when you actually checked-in, but it appears not.

I am coming back from SFo to LHR and at T-24 I tried online check-in to see if paid J to F upgrades were available. As there were none, I aborted check-in and left it to the airport. As it happens, SFO "haven't had the memo" as it was quaintly put and they don't offer them apparently.

I was surprised to see the SeqNo 002 on my bp, even though at least two other people checked-in before in the line.