The attribute grid can display anything that can be entered in a grid in numerical form. It is created in the same manner as any elevation grid. The input would be a position in site coordinates with the attribute data (instead of an elevation). The values are displayed numerically only. These are merely taken from the attribute grid (filename.grd) file and displayed alongside the text in the plan view.
It is for reporting a single number that has some value to the operator. In practice, when you select an attribute grid it asks you for attribute text; in one of the demo projects the text is Sulfur. So in the Grade screen the user will see: "Sulfur: 1.5" where 1.5 was the value queried from the attribute grid.
It is an extremely useful feature, although not frequently used. Furthermore, the use of this feature does require that the mine has some information that is of use and they take the time to model it.