IEC 61508 and IEC 61511 functional safety standards allow Safety Integrity Levels (SIL) to be used four ways by end users:

1. To establish risk reduction requirements - How much risk needs/can be removed from the process to achieve a tolerable risk level? (For more info see Risk Reduction.)
2. Probabilistic limits for hardware random failure - This is the random probability of a failure. This is calculated by PFDavg for low demand mode of operation or PFH for high or continuous demand modes (See PFDavg or PFH for more info.)
3. Architectural constraints - Sets rules for establishing the need of redundancy. (See Architectural Constraints for more info.)
4. To establish Systematic Capability - Systematic Capability is achieved either by choosing a certified part with SC to the given SIL level or greater or by completing a prior use justification to the given SIL level or greater. The lowest SC for any device in the SIF determines the SIL level for the SIF with respect to SC. (See Systematic Capability for more info)

For each of the ways to use SIL, there are tables and calculations in the standard. Some of these are shown below.