Hi Andrii
From a business perspective, I have trouble to imagine an order without any party related (but perhaps there are some UC that did not require it) - that why " Create Product Order API demands RelatedParty as mandatory".
But because, as you noted, relatedParty could be described at product order level or product level, I will be unconfortable with a cardinality 1..* at any level. It will limit flexibility to describe party at product order level only or at product level only in API implementations.
Best regards
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Ludovic Robert
Orange
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