Terms

PFD stands for "probability of failure on demand."
In the case of an SIS, a PFD of 1/1000 means that the system may fail once every 1000 times that it is required to function. In other words, by installing an SIS, the risk can be reduced to 1/1000.

This has the same meaning as the SIL risk factor. The lower the PFD, the higher the reliability and safety.