I think it's an open field for you to use however you want. E.g. If it's a Bill and you use the bill functionality, that column gets auto-populated for you using the "Bill ID". Likewise for sales-receipts, it gets populated with the Receipt ID that you filled in when creating the sales receipt. If you leave them blank when creating those items, an auto-incremented ID gets filled in. At least for Receipts and Bills, GNUCash keeps separate sequences of unique IDs.
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.