The MSC is used by many mathematics journals, which ask authors of research papers and expository articles to list subject codes from the Mathematics Subject Classification in their papers.
For example: At the top level, 64 mathematical disciplines are labeled with a unique two-digit number.
Physics (i.e. mathematical physics) is particularly well represented in the classification scheme with a number of different categories including: All valid MSC classification codes must have at least the first-level identifier.
Based on a scheme to organize AMS's Mathematical Offprint Service (MOS scheme), the AMS Classification was established for the classification of reviews in Mathematical Reviews in the 1960s.
This can sometimes make it difficult to search for older works dealing with particular topics.
Due to the large overlap between mathematics and physics research it is quite common to see both PACS and MSC codes on research papers, particularly for multidisciplinary journals and repositories such as the arXiv.
The classification scheme used on the arXiv is chosen to reflect the papers submitted.