The tax benefit ultimately goes to the shareholder. It would be in a corporation's interest to ensure that their shareholders get the maximum tax benefits and that they designate the maximum that they can as eligible dividends.
In the absence of some mechanism to stop them from doing so, they might designate even more than they were eligible for, and that's what this is about. This is to calculate any excessive dividend designation that may have been made in order to ensure that the corporation then has to pay up an amount to reflect the additional cost of the higher-rate dividend tax credit being claimed, or claimable, by the shareholder.