The Home Office provides a list of academic subjects that require an ATAS Certificate in the immigration rules. These are referred to as CAH3 codes.
Please see the full list of CAH3 codes for all subjects. This list provides the 'Overall Term' and a 'Definition' and confirms if ATAS clearance is required.
A project plan may fall under several CAH codes due to its interdisciplinary nature, for example medical research but involving artificial intelligence - computer science. All relevant CAH codes must be identified and if an aspect requires ATAS clearance, an ATAS application must be progressed. You are not looking for the 'best fit' under one CAH code.
The identification of the CAH code(s) applicable to the research project must be taken with the upmost diligence, and with input from the Principle Investigator/Supervisor directly involved with the research project.