Classification language. The composition of library information retrieval describes the characteristics of information content: classification language needs to be expressed in a certain language in order to make the retrieval process, that is, document identification and information questioning, go smoothly. Only with the help of this language can document indexers and retrieval personnel have a common agreement and communicate with each other. That is to say, the internal and external features of documents are described in a certain language when they are stored, and information questions are expressed in a certain language when they are retrieved. This kind of language is called retrieval language, which links the storage and retrieval of documents, and links indexers and retrieval personnel to achieve mutual understanding and communication.

Type of retrieval language:

According to the nature and principle of logo, it can be divided into three categories: ① classification language, ② theme language and ③ code language.

(1) Classified language refers to a retrieval language which takes numbers, letters or the combination of letters and numbers as basic characters, adopts the writing method of directly connecting characters and taking dots (or other symbols) as separators, takes basic categories as basic vocabulary, and expresses complex concepts with the subordination of categories.

Well-known classifications include International Decimal Classification, Library of Congress Book Classification, International Patent Classification Table, Chinese Library Book Classification and so on.

② Thematic language refers to a retrieval language with natural language characters, noun terms as basic vocabulary and a group of noun terms as retrieval marks. The information processing method of describing and expressing information content in topic language is called topic method. Topic language can be divided into headline words, meta words, narrative words and keywords.

(3) Code language refers to the characteristics of a certain aspect of things, and uses a certain code system to express and arrange the concepts of things, thus providing a retrieval language for retrieval. For example, according to the code language of compound molecular formula, a molecular formula index system can be constructed, which allows users to search the corresponding compounds and their related literature information from the molecular formula.