The reducing substances test detects sugars not normally present in urine and feces, such as glucose, lactose, maltose, fructose, and galactose. It serves as a screening tool for inborn errors of carbohydrate metabolism, helping to identify disorders associated with increased excretion of these sugars, including glucosuria and galactosemia. This test is essential for detecting non-glucose reducing substances and evaluating metabolic abnormalities.