ULD is an acronym of Unit Load Device. In order to facilitate loading/unloading of cargo on wide bodied aircraft, containers and pallets which fit the contour of the aircraft hull (upper hold for freighters and lower hold for all aircraft) had been designed.
By loading the containers and pallets on the ground, actual time required for loading/unloading have been greatly shortened allowing the aircraft to make a shorter stay on the ground. ULDs also serve as protection of the cargo versus loose loading.
CLOSED ULD means, containers which doors are sealed with tape, etc. not to allow free flow of air thus protecting unwanted germs and insects from entering into the country prior to plant quarantine.
OPEN ULDs are those containers and pallets with no sealing applied.DGR 18.104.22.168 refers to foodstuff loaded in CLOSED ULD and dangerous goods in another CLOSED ULD May be loaded adjacent to each other, whereas if they are on OPEN ULDs, they need to be loaded at a distant position.