Classifying mathematics is not an easy task. The American Mathematical Society sets up a classification aiming to cover most of (current) Mathematics. They a assign a code to each subject. This code is used by journals and search engines. The official link is the following.

https://mathscinet.ams.org/mathscinet/msc/

Some examples (from the 2020 version) are the following.

  • 91A05: GAME THEORY, ECONOMICS, SOCIAL AND BEHAVIORAL SCIENCES -> Game theory -> 2-person games
  • 90C31: OPERATIONS RESEARCH, MATHEMATICAL PROGRAMMING -> Mathematical programming -> Sensitivity, stability, parametric optimization
  • 90C05: OPERATIONS RESEARCH, MATHEMATICAL PROGRAMMING -> Mathematical programming -> Linear programming