If two persons, a creditor and a debtor, appear in court holding on to a note of indebtedness. The creditor says, "It is mine: it fell from me, and I subsequently found it," but the debtor says, "It was indeed yours. However, I repaid the debt to you," the witnesses who appear on the document must certify that the document is indeed authentic, and they divide it with an oath, just like in the case of a garment.
Why is the certification needed if the debtor admits the document is valid? Because if we believe him on that, we should also believe that he repaid, or else we need to confirm the document independently.
Art: Convention signed in London by Paul Cambon 1843-1924 the French Ambassador and Lord Salisbury 1830-1904 the British Prime Minister from Le Petit Journal 9th April 1899, by Tofani, Oswaldo Meaulle, F.L. & - Anglo French