The collection of books in the library is established according to certain purposes and functions. According to the requirements of use and function and the level of users, the collection can be divided into several grades, which can reflect the depth and integrity of the collected literature content and the level of meeting the demand for literature for a certain kind of work. Different countries have different standards and levels of collection classification, and different libraries in the same country have different practices. In the United States, books are divided into completion level, research level, learning level (including advanced learning level and primary learning level), basic level and lowest level. Library circles in China are generally divided into the following five levels:

A-level (complete level): Try to search all the documents of a certain subject (specialized field), regardless of content level, language, publishing form and book form, subject to complete collection. In fact, not all of them can be collected, but every effort should be made to reach this level for a certain kind of special collection.

Grade B (research grade): In order to meet the needs of independent research, it is necessary to collect all representative works of various schools in this field, including major foreign language works, anthologies, conference proceedings, core journals, reference books, etc. Scientific research institutions and majors or universities undertaking scientific research tasks should recruit books in the research fields designated by graduate students to reach this level.

Class C (university level): a wide range of basic periodicals, aiming at meeting the needs of college students and individuals for self-study of university courses, should collect all basic works, all works of important authors and related comments, excellent teaching materials, reference books, reference books and bibliographic materials. The related fields of scientific research institutions, the disciplines of university enrollment or preparation for enrollment, and all libraries that intend to help young people learn to reach the university level by themselves should have books in related professional fields at this level.

D-level (basic level): It is a selected book designed to introduce people to professional fields. Recognized representative works, basic teaching materials, reference books and representative periodicals should be collected, and foreign books and periodicals are not accepted in principle. Research institutions and university libraries should reach this level in their possible related fields; The books in the libraries of technical secondary schools and technical schools also belong to this level; Some professional books and periodicals that college students need to improve their cultural literacy and broaden their horizons also belong to this level.

E-level (lowest level): refers to the professional field outside the collection scope, and only a small number of very basic works or reference books are selected for emergency use.

Among the above five grades, Grade B, Grade C and Grade D roughly reflect the functions of the library and the level of users' needs, and basically adapt to the current professional education and personnel needs (high, middle and primary) and the depth of literature content in China. The complete level and the lowest level also reflect a special demand situation.