So when custodian prints gold notes to give someone loan, they are taking risk of that person reapproaching the custodian to give back the gold with that loan money itself. Now imagine all these gold notes are digital in our system. When Person A transfer 100 gold notes to Person B. The bank earlier owed 100 gold notes to Person A and now it owns 100 goldnotes to Person B. It was not the money that was transferred but instead "promise to pay you 100" was transferred.