Let's say there's a motor vehicle crash, the affected person flees from a roadblock, there's a pursuit, there's a crash, and someone dies. The RCMP are responsible for the investigation involving the affected person who was driving and who fled. The IIO is responsible for investigating whether or not the officer who initiated the pursuit committed an offence.
At the end of those two processes, the CPC—the commission for police complaints, because this is RCMP—is responsible for looking at whether or not the policies, the training, and the conduct of the officer in conducting a pursuit were appropriate.