Call numbers organize information into topics. It's the address where that particular resource lives within the library. The call number is placed on a label that attaches to the item.
The system divides all knowledge into twenty-one basic classes, each identified by a single letter of the alphabet. Most of these alphabetical classes are further divided into more specific subclasses.
For example, class N, Art, has subclasses
NA, Architecture;
NB, Sculpture,
ND, Painting;
as well as several other subclasses.
Individual topics are often broken down by general to specific topic (Impressionism), specific places (Germany), certain time periods (20th century), etc. Each topic is assigned a single number or a span of numbers.