GenerateID - This is the activity which is responsible for generating unique case id. Pega will be maintaining the case id instance details in Data-UniqueID table.
Case ID prefix will be playing major role while generating Case ID number. In Data-UniqueID table pyLastReservedID column pega will be maintaining the last case id (maximum number) which got generated inside system. While new case is getting created system will add +1 to the number which is currently available in pyLastReservedID column and it will assigned as a case id to the case.