Extra Flavor is a boost of the concentrated flavors in the specific e-juice you are boosting. If you like to vape with strong flavors or you are using a low power device, extra flavor is for you.
The extra flavor is added by us to the e-juice, in the mixing process.
For those who want to order our flavor concentrates and wants to calculate the total percentage of the flavor - The extra flavor adds another 20% flavor to the original amount. For example, if the amount of flavor is 25%, the extra flavor adds to it 20% from 25 = another 5%. So the total flavor now will be 25+5=30%