What is SIL1, SIL2, SIL3, SIL4?

Safety Integrity Level (SIL) is a measurement of performance required for a Safety Instrumented Function (SIF).

SIL is defined as a relative level of risk-reduction provided by a safety function, or to specify a target level of risk reduction.

Four SIL levels are defined, with SIL1 being the least dependable and SIL4 being the most dependable.