(From Iconclass website) The unique elements of the Iconclass system are its alphanumeric classification codes, called notations. Notations always begin with one of the digits 0 - 9, corresponding with the ten main divisions of Iconclass.

Notations can be used to describe or index the subjects of visual documents regardless of the complexity of the subjects. Several notations can be used to describe complex subjects.

Every notation is part of a hierarchical structure and, as such, incorporates all of its iconographical broader terms. The text explaining the meaning of an Iconclass notation is the notation's 'textual correlate'.

Main divisions of the Iconclass classification system
The main divisions of the Iconclass system are represented by digits 0 to 9. Of these ten 'main divisions', the numbers 1 to 5 are 'general' topics, designed to comprise all the principal aspects of what can be represented. Divisions 6 through 9 accommodate 'special' topics, coherent subject matter of a narrative nature, with an emphasis on the Bible (7) and Classical Mythology (9). A tenth division, represented by the number 0, was added in 1996 at the request of Iconclass users, to accommodate abstract art.

0 Abstract, Non-representational Art
1 Religion and Magic
2 Nature
3 Human being, Man in general
4 Society, Civilization, Culture
5 Abstract Ideas and Concepts
6 History
7 Bible
8 Literature
9 Classical Mythology and Ancient History

Subdivisions: Increasing specificity
Within each division of Iconclass, definitions are organized according to a logic of increasing specificity. A main division is divided further into a maximum of nine subdivisions by adding a second digit to the right of the first one. Division 2 Nature, for example, is subdivided in the following way:
21 the four elements, and ether, the fifth element
22 natural phenomena
23 time
24 the heavens (celestial bodies)
25 earth, world as celestial body
26 meteorological phenomena
29 surrealia, surrealistic representations

The third level of specificity is attained by adding a letter in upper case. For reasons of legibility in the original printed version of Iconclass, the letter 'J' was omitted. Thus the addition of a letter permits as many as 25 subdivisions instead of 9, and it also increases the legibility of the notations.*)

*) Blank spaces on either side of the capital letter and between successive pairs of digits also provide greater legibility. However the implementation of notations with 'spaces' can be rather demanding in computer projects and has therefore been abandoned in many computer projects.

Take a look, for example, at the subdivisions of 25 earth, world as a celestial body:
25A  maps, atlases
25B  continents represented allegorically
25C  geological phenomena
25D  rock types; minerals and metals; soil types
25E  geological chronological division; historical geology
25F  animals
25G  plants; vegetation
25H  landscapes
25I   city view, and landscape with manmade constructions
25K  landscapes in the non-temperate zone, exotic landscapes
25L  cities represented allegorically or symbolically
25M  the Seven Wonders of the World
25N  fictitious countries

From the letter(s) onward, all subsequent descents in the hierarchy take place by extending the notation to the right with more digits.

The following example shows the first subdivision of 25F animals:
25F1 groups of animals
25F2 mammals
25F3 birds
25F4 reptiles
25F5 amphibians
25F6 fishes
25F7 lower animals
25F8 extinct animals
25F9 misshapen animals; monsters

Hierarchical path
The following example from division 7 Bible shows the hierarchical principle: all subsequent descents in the hierarchy take place by extending the notation to the right with more digits:
7 Bible
71 Old Testament
71H story of David
71H7 David and Bathsheba (2 Samuel 11-12)
71H71 David, from the roof (or balcony) of his palace, sees Bathsheba bathing
71H713 Bathsheba receives a letter from David
71H7131 Bathsheba (alone) with David's letter