Design essentials to achieve a specified SIL

The safety function is a cornerstone of functional safety .. get it wrong and you may have no protection - The SIL of a safety function is a cornerstone of functional safety??.if it is too low you increase the risk ?? The SIL concept was developed as a parameter to determine the rigour of the measures to be applied to combat systematic failure causes ?? The SIL is the key parameter for the Architectural Constraints (HFT) and the Systematic Safety Integrity measures (H/W & S/W) ?? The mode of operation a key aspect of SIL determination