Curtain
The curtain is the complete assembly of interlocking slats that forms the moving surface of a UK roller garage door, winding onto the barrel as the door opens.
The curtain runs within side guide channels to keep it aligned as it travels. Damage to the curtain, rather than the barrel or motor, is the most common source of roller door faults.
UK vs US usage
US "rolling steel door" or "coiling door" content rarely uses "curtain" as the term for the moving slat assembly — UK trade usage consistently calls it the curtain.