According to GB3469-83 "File Type and File Carrier Code", it is 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

For electronic documents with non-paper carriers, the carrier type shall be indicated in the reference mark:

A database on an online network

Database/tape database

M/CD-CD books

CP/DK disk software

J/OL- online log

EB/ol- online electronic bulletin