Due to the large size of roof boxes they are generally vulnerable to damage when being stored. The 6983 is a convenient slip on cover which helps prevent scratching and marking during storage. It also keeps your box clean and tidy. The 6983 is the largest size box cover which works with the Thule 820 and 900 series boxes. It is also a nice fit for Yakima Skybox 18 and 21 roof boxes.