The Joint Academic Coding System (JACS) is a way for UK Higher Education Institutions to classify subjects and modules. Codes are used for all programmes and modules at Undergraduate, Postgraduate Taught and Research level. At Undergraduate level UCAS codes are based on JACS codes.
Read more about JACS codes here: https://www.hesa.ac.uk/jacs3
Find a full list of JACS codes here: https://www.hesa.ac.uk/content/view/1787/281/