It's relevant to the US rather than the UK.
In the US carbs and fibre are included in the total carbohydrate count.
A label will say
carbs...
of which fibre...
To find the net carbs you subtract the fibre.
Here in Europe labels will list carbs and fibre as completely separate item. The fibre content has already been deducted from the carbohydrate content.
ie the carbs as listed on a European label are net carbs already..
It may be of importance to subtract fibre if you use US sites for nutrional information.