Staff classifications and the mapping between category and sub-category is described in the document: Staff Classification Guide
Staff classifications are assigned at the staff request stage of the appointment process - How-To guide - Create and approve Staff Requests. If basing a staff request on an existing post be aware that historical data anomalies are present in posts within CoreHR - check that the staff classification is accurate and complies with the guide every time you raise a staff request.
Check staff classification validity
Classification validity can be checked for permanent, fixed-term, open-ended and variable hours contract by running the HRINFO01_Data Quality Validation report monthly.