The serial number of a periodical is the ID number given to each periodical by the State Administration of Press, Publication, Radio, Film and Television.

For example, I just found out on the paper submission platform of China academic journals:

Contributed by China Journal of Biomedical Engineering CN11-5668/RISN1674-1927.

Contributed by cn10-1218/rissn1001-2379 journal of traditional chinese medicine.

Contributed by China Medical Science CN11-5547/RISSN1673-7555.

China journal of traditional chinese medicine information cn11-3519/rissn1005-5304 contributed.

Cn21-1546/rissn1673-7717 in journal of traditional chinese medicine, China.

In the above article, cn stands for the standard serial number of China, issn stands for the periodical number in the international standard, and sibn stands for the international standard book number (ISBN for short), which is an international number specially used to identify books and other documents.