Customs duty is calculated on the determination of the assessable value in case of those items for which the duty is levied ad valorem. This is often the transaction value unless a customs officer determines assessable value in accordance with the Harmonized System. However, for certain items like petroleum and alcohol, customs duty is realized at a specific rate applied to the volume of the import or export consignments. Harmonized System of Nomenclature For the purpose of assessment of customs duty, products are given an identification code that has come to be known as the Harmonized System code. This code was developed by the World Customs Organization based in Brussels. A Harmonized System code may be from four to ten digits. For example, 17.03 is the HS code for molasses from the extraction or refining of sugar. However, within 17.03, the number 17.03.90 stands for "Molasses (Excluding Cane Molasses)". Introduction of Harmonized System code in 1990s has largely replaced the Standard International Trade Classification (SITC), though SITC remains in use for statistical purposes. In drawing up the national tariff, the revenue departments often specifies the rate of customs duty with reference to the HS code of the product. In some countries and customs unions, 6-digit HS codes are locally extended to 8 digits or 10 digits for further tariff discrimination: for example the European Union uses its 8-digit CN (Combined Nomenclature) and 10-digit TARIC codes.