MINC variables fall into one of four classes. The first is the group class, which define data objects and names which are used to hold actual image data and supporting information. The second is the var-attribute class, which contain data qualifying another variable, for example, by specifying the range of the other variable's data. The third and fourth are the dimension class and the dimension-width class, which are used to specify the properties of the dimensions in a MINC file.