The Dewey Decimal System, often used by schools and public libraries, is based on ten general topics represented by numbers:

000 Generalities
100 Philosophy
200 Religion
300 Social sciences
400 Language
500 Natural sciences & mathematics
600 Technology (Applied sciences)
700 The Arts
800 Literature & rhetoric
900 Geography & history

These general categories are further subdivided into more specific topics. For example, the book Teleworking has the call number
331.25 T269 1998:

300 - Social Sciences
330 - Economics
331 - Labor Economics
331.2 - Compensation and other conditions of employment
331.25 - Including telecommuting