Glycemic index of a starchy food is a measure of its:
The increase in blood glucose after a test dose of carbohydrate compared with that after an equivalent amount of glucose (as glucose or from a reference starchy food) is known as the glycemic index. 
Glucose and galactose have an index of 1 (or 100%), as do lactose, maltose, isomaltose, and trehalose, which give rise to these monosaccharides on hydrolysis. 
Fructose and sugar alcohols are absorbed less rapidly and have a lower glycemic index, as does sucrose. The glycemic index of starch varies between near 1 (or 100%) and near 0 as a result of variable rates of hydrolysis, and that of nonstarch polysaccharides is 0. 
Foods that have a low glycemic index are considered to be more beneficial, since they cause less fluctuation in insulin secretion.

Ref: Harper’s illustrated biochemistry, 31st edition.
Energy
Utility
Collectivity
None of the above