IEC 62443

Nominating the standard is one of the highest-value things you can do in a brief. A design produced to the wrong code is not a design that needs adjusting — it is work that has to be redone, and the cost of finding that out late is entirely avoidable.

State IEC 62443 and your jurisdiction explicitly. The engineer should confirm the choice before starting and record it in the design basis, so every downstream check is traceable back to the clause it was made against.