Well, no, but there is no exact answer to the question.

At the base of it, a calorie is a calorie. 170 calories of any food is 170 calories. The measurement "calorie" is simply a unit of energy.

What net carbs does is discount those calories due to the claim that the body doesn't really digest them. However, as mentioned, the basis behind this is new as far as nutritional science goes and hasn't been studied too well. Labels can use net calories on their count because the FDA has no rules for them. There also would need to be a study on every single item labeled as a net calorie separate (fiber vs alcohol sugar vs allulose, etc).

If net calories DO work as they say, then that Quest bar is 170 units of energy to your body as it cannot use the other 81. If they don't, well, it was 251 after all. Or, perhaps more likely, it's somewhere in the middle.

There's also the whole issue of, even if they don't count as energy, does it effect the sugar levels of diabetics? (And yes, some sugar alcohols are known to do this despite being a net carbs.)