For IEC61511, SRS is normative requirement
The objective of SRS is to specify the requirements for the SIS, including any application programs and the architecture of the SIS.
The aim of SRS is to have every SIF described in such a way that anyone wherever and whenever need to understand the SIF;
The safety requirements shall be derived from the allocation of SIF and from those requirements identified during H&RA. The SIS requirements shall be expressed and structured in such a way that they are