Safety Instrumented Systems are designed to provide protection of life and property. Their design is based on risk evaluation procedure, use of voting and redundancy as well as using components that have a record of performance.
WiKi:
"Safety Integrity Level (SIL) is defined as a relative level of risk-reduction provided by a safety function, or to specify a target level of risk reduction. In simple terms, SIL is a measurement of performance required for a Safety Instrumented Function (SIF).
Four SILs are defined, with SIL4 being the most dependable and SIL1 being the least. A SIL is determined based on a number of quantitative factors in combination with qualitative factors such as development process and safety life cycle management. The requirements for a given SIL are not consistent among all of the functional safety standards."
Two main Standards identify the core: IEC 61508 and ISA S84.
It is mainly used in the Process Industry, but its principles of a methodical approach have universal use.