In functional safety, safety integrity level (SIL) is defined as the relative level of risk-reduction provided by a safety instrumented function (SIF), i.e. the measurement of the performance required of the SIF.[1]
In the functional safety standards based on the IEC 61508 standard, four SILs are defined, with SIL4 being the most dependable and SIL1 the least. The applicable SIL is determined based on a number of quantitative factors in combination with qualitative factors, such as risk assessments and safety lifecycle management. Other standards, however, may have different SIL number definitions.[2]