Document type identification
It is a symbol representing various types of references. The description of references should comply with GB 77 14-87 "Rules for the Description of References after Articles" and "Data Specification for Retrieval and Evaluation of Academic Journals in China (CD-ROM Edition)". The author of the paper should use the following document type identification codes to indicate the types and carrier types of various references cited by himself.
According to GB 3469-83 "File Types and File Carrier Codes", common file types are identified with single letters, and electronic files are identified with double letters.