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Basic Functional Safety Question

Hi all, I received this non-SIS specific question in an email that I wanted to open up to the EE365 community for your thoughts... thanks!

As an SIS integrator, I was confused about the failure rate calculation of the SIS system. Clients always ask for a failure rate calculation for the configured SIS when the sensor and final element are not in supply scope of the SIS integrator.

An SIS always consists of many SIFs in a single station, and the SIL verification shall be based on a single SIF, which means only the components used in the SIF loop shall be included in the PFD calculation.

I suppose that the failure rate which client asked for is to verify the SIL of SIS and to be used for SIF loop verification. Then the question is, should I calculate the SIS failure rate based on all safety related components including CPU, IO modules and barrier/relay which installed in the SIS, or just simply calculate the components which covered by a most complicated SIF?