Suppose a corporation had the ability to pay dividends out of income that was taxable at the high rate of $1 million, but they had some other sources of income taxable at the low rate. They actually would have the capability of paying a $1.5 million dividend. Under these rules they should pay a half-million-dollar dividend as a regular dividend that is eligible for the low-rate tax credit, and they would have $1 million that they could pay as a dividend eligible for the high-rate dividend tax credit.
But it may be that they designate the whole thing as eligible for the high-rate dividend tax credit. Maybe it's a mistake, or maybe it's otherwise. If they do so, the shareholder will be eligible for the high-rate dividend tax credit. They don't have to look behind what the corporation has told them.
In this circumstance, the corporation would have had an excessive eligible dividend designation of a half million dollars because they designated half a million dollars too much, and they would have to pay a charge to offset the fact that the shareholder is getting a higher dividend tax credit on that dividend than is warranted.