Classification Principles
The ICS uses an hierarchical classification, which consists of three nested levels called fields (Level 1), groups (Level 2) and sub-groups (Level 3). Each field is subdivided into groups, which are further divided into sub-groups.
All classification levels are designated by a classification code (called notation) and a title. The notation is a set of Arabic numerals.
Top-level items, which have no parent levels, use a two-digit notation, for example:
43 ROAD VEHICLE ENGINEERING
The notations for groups and sub-groups include the parent-level notations. The example below shows a notation for Sub-Group 20 (Level 3), which belongs to Group 040 (Level 2) in Field 43 (Level 1).
43.040.20 Lighting, signaling and warning devices
Read more about this topic: International Classification For Standards
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—Eugene V. Debs (1855–1926)