To determine the Custom Code Impact Cases, follow these steps: 1. Identify the areas where custom code is implemented within the system. 2. Analyze the dependencies and interactions of the custom code with standard system functionalities. 3. Evaluate the potential impact of system updates or changes on the custom code. 4. Document the findings and categorize the impact cases based on severity and priority. 5. Develop a plan to address the identified impact cases, including testing and validation strategies.