At Level 0 there are no automated driving functions. The driver performs the longitudinal control of the vehicle (i.e. maintaining speed, accelerating, and braking) and the lateral control (i.e. steering). There are no systems that intervene, only those that issue warnings.
At Level 1 a system can assume either longitudinal or lateral control of the vehicle, while the driver continuously performs the other task.
It is only at Level 2 that one speaks of partial automation, because the driver can now relinquish both tasks, i.e. longitudinal and lateral control, to the system in a certain use case. The driver continuously monitors the vehicle and the traffic during the journey. At all times they must be in a position to resume control of the vehicle immediately.
At Level 3 the system independently recognizes its limits, that is, the point at which its functions can no longer cope with the environmental conditions. In this case, the vehicle requests the driver to resume the task of driving. The driver no longer has to continuously monitor the longitudinal and lateral control of the vehicle. However, he must be able to resume driving when the system signals him to do so, with some extra time in reserve.
From Level 4 onwards, the driver can hand over the entire task of driving to the system in specific use cases. These scenarios refer to the type of road, the speed range, and the environmental conditions.
The last development level is that of driverless driving, Level 5. The vehicle can completely independently perform the task of driving in full on all types of roads, in all speed ranges and under all environmental conditions. At present nobody can say when this level of automation will be achieved. The research and development is initially focusing on the automation levels for partially, highly, and fully automated driving. Fully automated driving on highways will probably become possible in the decade after next.