Impact carbs (or net carbs) are only those carbs that affect blood glucose level. Therefore, in diets with carb counting, only carbs that raise blood sugar are counted.
Impact carbs raise blood sugar, like starch and sugar. Other carbs do not raise blood sugar or do so to an insignificant degree. The first of these carbs is fiber. This carb comes from fruits and vegetables and moves through the digestive system without becoming dissolved and being broken down. Because it does not dissolve, it is not broken down into glucose and does not raise blood sugar levels.
Sugar alcohols are also not normally counted in impact carbs because they also do not raise blood sugar levels, About.com explains. However, this depends largely on the type of sugar alcohol. There are three types, maltitol, sorbitol and erythritol, and each one affects blood sugar differently. Malititol raises blood sugar most significantly, which is then added to net carbs. Erythritol has the most insignificant impact on blood sugar levels and is not counted in net carbs.
About.com states that there are some carbs scientists, such as glycerine, polydextrose, oligofructose and inulin, on which scientists have very little information. Although glycerine can be used to make glucose, it also looks to have a low impact on blood sugar. Inulin also tends not to dissolve in the digestive tract.