Are you supposed to keep that globally unique or just for each verification you log? Because if it's only a local sequence for each verification you make then there's bound to be overlap between account views.
And if it's globally unique then I'd like GnuCash to automatically increment it.
Looking at this page on gnucash.org now (1), it's referred to as just "optional Num (such as a check number)". Hope that helps.
[1] http://gnucash.org/docs/v2.4/C/gnucash-guide/txns-registers1...
So, you can use it to match it against a bill, invoice, whatever is applicable to that entry.
Also, had a quick look in the account XML regarding Bill/Receipt ID. It get's stored as ID, even though the bill/receipt has a unique GUID to identify it. I would assume that means it need-not be unique.