Bonds can vary in price (What causes bond prices to move?) and Capital Gain / Loss is the difference between a Fractional Bond's purchase price and its selling price, when the difference is positive / negative.

Portfolio Value in your Portfolio Summary shows the estimated value you could get for selling the Fractional Bonds in our Secondary Market.

Bonds have to be repaid by maturity, so the amount of money you are owed does not change as the bonds' prices fluctuate.