The article discusses how securities collateral can be replaced with other collateral during trades. This usually happens when the bonds are needed for other transactions like selling securities or lending them. The process of substitution is described, including the steps involved. If the replacement collateral meets certain criteria, the party receiving the collateral must accept the substitution request. Both parties need to be careful during this process to avoid creating unwanted risks. It's important to update the total collateral balance after substitution to accurately calculate ongoing exposures.