A nice article on this...
http://women.webmd.com/features/net-carb-debate

And from Atkins site -

When you follow the Atkins Nutrition Approach, you actually count Net Carbs, which means the total carbohydrate content of the food minus the fiber content. The Net Carb number reflects the grams of carbohydrate that significantly impact your blood sugar level. These are the only carbs you need to count when you do Atkins. Foods that are low in Net Carbs are foods like nutrient-dense vegetables and fruits that don’t have a significant impact on blood sugar and therefore don’t cause you to gain weight. Sometimes we call these foods “good carbs.”

You can calculate the approximate Net Carbs yourself by looking at the information provided on a food label (grams of total carbohydrates minus grams of fiber). For foods that don’t have a label, like fruits and vegetables, you can use the Atkins Carb Counter.

Atkins science allows us to calculate Net Carbs in our products more accurately. In addition to subtracting grams of dietary fiber from total carbohydrates, we’re able to account for glycerin and other ingredients that have minimal impact on blood sugar levels that might not show up on a standard food label. We can also check Net Carbs using analytical techniques. But what is important for you to know is that all Atkins bars and shakes are low in Net Carbs.

I think subtracting fiber is mostly valid. But there are some questionable products out there (I'm talking about you, low carb tortillas) where things often don't add up. I really question having 30 grams of carbs with 28 grams of fiber. My suspicion is that they are adding sources of fiber to the ingredients, and counting it in the fiber count without counting it in the carb count. I have no data to back this up. It just seems off to me. (Low Carb Tortillas are a YMMV thing -- if you have a stall, they are the first thing you should be cutting).

The second class that you typically subtract is sugar alcohols. However, sugar alcohols are a broad class of items - a number of which DO have impact on your blood sugar. In my opinion, erythritol is one that is OK to completely subtract - glycemic index is zero. Malitol is a horrid product that has a relatively high glycemic index. If it's malitol in your product (and now I'm talking to you, Atkins bars) you should count some of those sugar alcohols in your carb count. I think a lot of people count half the Sugar Alcohols. Atkins Bars are another YMMV item - be cautious about eating these too regularly. And if you eat too many, you will pay. That malitol will make itself known (be near a bathroom).

Mark's Daily Apple had a good article on some of the different sugar alcohols --
http://www.marksdailyapple.com/sugar-alcohols/#axzz2jiSLfSLk

I think the labeling in Europe is different, this may also confuse the issue if you are not in the United States.

You can't go wrong if you include the entire carb count in your allowance, you'll actually be eating less than your limit and for some people that's good! Just know that you could potentially eat more than that and still stay within your limit.