Classifications

 UDC — Universal Digital Classification,
https://www.triumph.ru/html/serv/udk.html?category_id=16169
"Universal Digital Classification (UDC) is a basis for systematization of human store knowledge in libraries, data bases and other informational stores. UDC is accepted to categorize documents in the majority of countires in the world. UDC is an obligatory component in Russia of all publishing production and information on natural and technical sciences"

 MSC 2020 — Mathematics Subject Classification
https://mathscinet.ams.org/msc
The Mathematics Subject Classification (MSC) is used to categorize items covered by the two reviewing databases, Mathematical Reviews (MR) and Zentralblatt MATH (Zbl). The MSC is broken down into over 5,000 two-, three-, and five-digit classfications, each corresponding to a discipline of mathematics.

The current 2020 Mathematics Subject Classification (MSC2020) is a revision of the MSC2010.

 PACS^® — Physics and Astronomy Classification Scheme®
http://www.aip.org/pacs/
The Physics and Astronomy Classification Scheme® (PACS^®) is prepared by the American Institute of Physics (AIP) in collaboration with certain other members of the International Council on Scientific and Technical Information (ICSTI). PACS is a hierarchical subject classification scheme designed to classify and categorize the literature of physics and astronomy. PACS provides an essential tool for classification and efficient retrieval of literature in physics and astronomy; as such, PACS is used by AIP and other international publishers of journals in physics, astronomy, and related fields.