Without getting in to detail, it comes from the freighter crewing agreement formed a number of years ago. Pilot's at the time had the option of opting out of freighter flying. However, more recently they have changed the cos for new hires to say that you may be required to fly the freighter. Although the company can roster you to fly the freighter as an SO, I don't believe this is a common occurrence. When you upgrade it will be on the 330 or 777, again not an issue for freighter flying. The only time this will come up is if you transfer over to the 744 as an FO or Captain.