According to GB3469-83 "File Types and File Carrier Codes", reference file types and file types are identified by a single letter:
Monograph m; Newspaper n; Journal j; Patent document p; Compilation g; Ancient books o; Technical standards;
Dissertation d; Scientific and technological report; Reference tool k; Search tool w; File b; Tape a;
Chart q; Record l; Product sample x; Video tape v; Meeting minutes; Chinese translation test;
Music score I; Movie y; Manuscript h; Microfilm u; Slide z; Miniature flat film f; Other e.