13.8. The HS12 structure and the classification scheme. The HS is a structured nomenclature comprising a series of four-digit headings, most of which are further subdivided into five- and six-digit subheadings. The 2012 edition of the HS comprises 5,205 groups of goods identified by a six‑digit code (compared with 5,052 in the 2007 edition) and is provided with the necessary definitions and rules to ensure its uniform application. HS12 comprises a total of 1,224 headings which are grouped in 96 Chapters, the latter being themselves arranged in 21 Sections. The headings are identified by a four‑digit code, with the first two digits indicating the Chapter in which the heading appears (a leading zero is used with the first nine Chapters) and the second pair of digits referring to the position of the heading within the Chapter.
13.9. The general structure of HS12 is as follows:
Sections I to IV: Agricultural products
Sections V to VII: Minerals, chemical and related products, plastics, rubber and articles thereof
Sections VIII to X: Animal products, such as hides, skins and furskins, as well as wood, cork, pulp, paper, and articles thereof
Sections XI and XII: Textiles, footwear and headgear
Sections XIII to XV: Articles of stone, plaster, cement, asbestos, mica and the like, ceramic products, glass, pearls, precious or semi-precious stones, precious metals, jewelry, base metals and articles thereof
Section XVI: Machinery, mechanical appliances and electrical equipment
Section XVII: Vehicles, aircraft, vessels and associated transport equipment
Section XVIII: Optical, photographic, cinematographic, measuring, checking, precision, medical or surgical instruments and apparatus, clocks and watches, musical instruments
Section XIX: Arms and ammunition
Sections XX and XXI: Miscellaneous manufactured articles, such as furniture, lighting fittings, prefabricated buildings, sports requisites, works of art, collectors’ pieces and antiques
13.10. Causes of amendments contained in HS12. Environmental and social issues of global concern are the major feature of the HS12 amendments, particularly owing to the use of the HS as the standard for classifying and coding goods of specific importance to food security and the early warning data system of the Food and Agriculture Organization of the United Nations (FAO). The volume of amendments within, for instance, Chapter 3, for the separate identification of certain species of fish and crustaceans, mollusks and other aquatic invertebrates, is substantial. The modifications aim at improving the quality and precision of trade data in these commodities. The amendments include, inter alia, improved specifications for species from the southern hemisphere. These amendments will enable economic trends in products other than those familiar to North Atlantic consumers to be monitored. In the same vein, new subheadings have been created for the separate identification of certain edible vegetables, roots and tubers, fruit and nuts, as well as cereals. The HS12 also features new subheadings for specific chemicals controlled under the Rotterdam Convention of the Prior Informed Consent Procedure for Certain Hazardous Chemicals and Pesticides in International Trade and ozone layer depleting substances controlled under the Montreal Protocol on Substances that Deplete the Ozon Layer. Other amendments resulted from changes in international trade patterns. These include deleting 43 subheadings due to the low volume of trade in specific products, separately identifying certain commodities in either existing or new headings, and reflecting advances in technology where possible. Finally, a number of amendments aim to clarify texts to ensure uniform application of the HS.
13.11. Reuse of codes. Whenever revisions are made to the HS, some existing items are deleted and new items are added by the creation of new headings (four-digit codes) or subheadings (six-digit codes). In order to accommodate users who maintain data under different versions of the HS, code numbers for commodities that have been deleted are not re-used until a certain period has elapsed, unless reuse is unavoidable. Where possible, compilers are encouraged to follow the same practice for the more detailed commodity codes used in national commodity classifications.
13.12. Implementation of the HS12: correlation tables. The WCO Secretariat has issued the correlation tables between the 2012 and 2007 versions of the HS, and updated HS publications, such as the Explanatory Notes, the Compendium of Classification Opinions and the Alphabetical Index. Customs administrations have the serious task of ensuring timely implementation of HS12, as required by the HS Convention. Trade data compilers are advised to cooperate with national customs administrations in ensuring that data collection in terms of HS12 is carried out on time.
 HS Chapter 77 is reserved for possible future use and HS Chapters 98 and 99 are reserved for special use by contracting parties. Countries should avoid, where possible, the use of Chapters 98 and 99.
 United Nations, Treaty Series, vol. 2244, No. 39973.
 Ibid., vol. 1522, No. 26369.