the window seat has full under-seat storage space. the aisle seat does not. this center under-seat section should first be reserved first for personal item storage of the person in the aisle seat (assuming it isn't a tiny bag that fits in the tiny space next to the aisle). If the person in the aisle seat doesn't have an under-seat personal item, I think it is generally ok for the window seat person to use the middle space. In this pictured instance, I would turn the briefcase sideways to leave the aisle seat person a little leg room in the middle section.