Staff classifications are assigned at the staff request stage of the appointment process (RQ1–Creating a staff request guide). If basing a staff request on an existing post be aware that historical data anomalies are present in posts on Core - check that the staff classification is accurate and complies with the guide every time you raise a staff request.
In some cases it can be difficult to determine if a role is undertaking academic research and should be assigned to a Category 2 'Research only' code. Some in-depth guidance to help you decide is availablehere. The advice from HESA is that research must be 'generating new knowledge'.