The three constraints (systematic capability constraint, architectural constraint, and probabilistic performance metric constraint) that are implied by requirements per international safety standards IEC 61511 [1] and IEC 61508 [2] to determine the safety integrity level (SIL) of a safety instrumented function (SIF) are described and discussed. Examples of their applications are presented. For low demand mode SIF operation, the importance of including numerous key variables in the computation of average probability of failure on demand (PFDavg) is noted.