Sales Factor Apportionment is a corporate tax that only taxes corporate profit. It determines what profit is taxable in the U.S. by determining how much of the profit can be attributed to the U.S. Instead of relying on labor or capital which can be moved easily, the profit is compared to the sales which are hard to move. Any attempts to move sales is on paper and can be easily quashed.
To give up being taxed in the U.S., any corporation has to give up market share in the U.S. So the total profit of a corporation will be multiplied by the fraction of U.S. Sales as a part of total sales. This result is the corporate profit made in the U.S.