Swing doors are the two hinged doors at the back of a trailer that open outward from the middle. They seal tightly and are sturdy, which is why they’re standard on reefers, where keeping temperature in matters, and common on dry vans.
The trade-off versus a roll-up door is clearance: swing doors need open space behind the trailer to swing wide, which can be awkward in tight yards or when backed close to another vehicle. In exchange they give a better seal and hold up well.