A case, is a way of grouping together sessions that belong together.
If a client comes to see you repeatedly following a car accident, you could for instance create a case named "Motor Vehicle Accident". You would then include all related sessions inside that case.
If the same client comes to see you two years later following a work accident, you could create a new case named "Work Accident" for the same client. You would create all new sessions within that new case.
If you are a Massage Therapist providing massages that are not meant to treat any specific condition, create a case named "Wellness". Group all wellness sessions under this case.