According to GB3469-83 File Types and File Carrier Codes, various commonly used files are identified by a single letter:
M- Monographs (including works on history and chronicles in ancient books)
Essays
Newspaper articles
Journal articles
Doctoral dissertation
R- research report
S standard
patent
A—— Precipitated documents in monographs and papers.
Z—— Other unspecified literature types
Electronic document types are identified by two letters:
database
computer program
Electronic bulletin