You mean [J] or [S] in the list of references at the end of the paper, right?

This is the reference type identification code, which is recorded in the reference and indicates the type of reference, marked with [], including M (book), J (journal), D (dissertation), S (standard), P (patent), N (newspaper), C (conference paper), R (report) and so on.

So when you ask [J], it means that the reference is a journal, and [S] means that the reference is a standard.

—— Refer to GB/T 77 14-20 15 "Information and Literature Reference Description Rules".