Sorry, but I don’t understand why you would want to keep speculating. Either your customer comes clean and explain the source of the mandate or not.
good luck.
My customer is a large company and like most companies of any size there are diverse agendas, goals. departments and the like. My guess is that the people I deal with don't know exactly why the mandate is there and I doubt they could give me a detailed explanation, it's not their area of expertise either.
They know what they've been told and probably aren't in any better position to evaluate it than I am; and probably, and understandably, less motivated than I am to sort it out.
My hope in speculating/researching was to give them something to work with, as I said something to push back with. I don't have access to whoever made this decision.
And as I've found out, while researching and speculating, the situation is much more complicated than whether or not 13485 requires that their suppliers be ISO 9001 certified. Good that I didn't just go to them with that as if that settled the matter.
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Good luck to you too.