Many thesauri, such as NALT, the General Finnish Ontology (YSO), UNESCO Thesaurus, and the STW Thesaurus for Economics (Germany) provide a thematic division of concepts into clusters orthogonal to the hierarchy of broader and narrower concepts. Thematic categories allow users to view concepts by subject area or discipline and present specialists with concepts of interest in their fields.
In creating GACS, the GACS Working Group re-used a classification scheme developed by CABI, FAO, and NAL in the 1990s for a joint project, the Unified Agriculture Thesaurus (UAT). A version of CABT using the classification scheme was published in 1999 as the CAB Classified Thesaurus in a single edition. The 1999 edition was found to cover over 80% of the concepts in GACS, and the remaining 20% of concepts were classified manually.
The classification scheme, which is based primarily on the CAB Classified Thesaurus, is specific to GACS and may be pruned or expanded in accordance with GACS requirements.
Headings in the classification scheme are designated with two-letter codes.
In addition, each heading is identified with a URI in the GACS namespace,
such as http://id.agrisemantics.org/gacs/G_HW
HW organic compounds.
The upper level has five categories:
Code Classification Heading CA GENERAL FA PHYSICAL SCIENCES JA EARTH SCIENCES LA LIFE SCIENCES PA APPLIED SCIENCE AND TECHNOLOGY WA SOCIAL SCIENCES AND HUMANITIES
The five categories in the top-level classification serve to cluster the categories of the lower levels and are not themselves intended to be used for indexing. The second level of the hierarchy has 30 categories, the third 91, and the fourth 14, with four categories on a fifth level. Scope notes for the classification prepared by Lori Finch include notes on how lesser-used categories might be combined or deleted.
Each concept in GACS is assigned at least one category. The classification is not strict, and terms can be placed in more than one category.