An INFRASTRUCTURE BOND is a municipal debt instrument used to fund public works. The city issues the bond at a slight premium, then redeems it for cash when the project is complete.
Once your empire owns ADAM SMITH'S TRADING HOUSE, bond cities stop completing and instead pool their shields each turn for any other city in the empire to draw on.
Cities discovered early that public works could be financed by future tax revenue. The bond market made aqueducts, roads, and harbours possible without royal subsidy. Investors earned a modest premium; citizens gained lasting infrastructure.