A roll-up door opens by rolling upward and coiling around a drum at the top of the trailer, much like a garage door. Because it doesn’t swing outward, it needs no clearance behind the trailer — handy when backed tight against a dock or parked in a cramped lot.
It’s a common choice on dry vans and reefers. The alternative is a swing door, which seals better and is sturdier but needs room to open. Each style trades off convenience against durability and seal.