How do you allocate SIL requirements across subsystems of a complex SIF?

SIL allocation for complex SIF: overall SIF PFDavg target distributes across sensor, logic, and final element subsystems. Allocation approaches: equal distribution (each subsystem contributes 1/3 of target PFDavg), proportional to complexity (simpler elements have higher PFDavg allocation), or constrained by available technology (valves may limit, allocate remainder to sensors/logic). Verify allocation: sum of subsystem PFDavg must not exceed target, each subsystem must meet architectural constraints for allocated SIL. Document allocation in SRS with rationale. During detailed design, rebalance allocation based on actual component data. Critical: avoid over-specifying one subsystem while under-specifying another. Consider design flexibility for future modifications.