The term Performance Level was coined by ISO and stands for the average probability of a failure of safety-relevant systems.
The performance level of robots is described by the classes d and e. Class d means that the probability of failure is between `10^-7` and `10^-6`. This means that class-d robots fail every `10^6 and `10^7` hours. For class-e robots, the values are one power of ten higher, i.e. between `10^7` and `10^8` hours.
These are the minimum performance levels specified in ISO 10218-2.