Classification is a method of systematically organizing and revealing information by using literature classification as a tool according to the subject knowledge content and other remarkable characteristics reflected by literature. Commonly used document classification methods include: China Library Classification, Renmin University of China Library Classification, China Academy of Sciences Library Classification, China Archives Classification, Dewey Decimal Classification, International Decimal Classification, Library of Congress Classification, Colon Classification and Bliss Bibliography Classification.
Theme method, also known as theme language. It is a retrieval language which takes words in natural language as characters, standardized or non-standardized noun terms as basic vocabulary, formal logic between concepts as grammar and word formation, words in order, and subject concepts as reference system to express the relationship between concepts. Commonly used languages include headline language, meta-word language, narrative language and keyword language. According to their combination characteristics, they can be divided into the first group, the second group and the dispersed group.