You are following the BKP scope item and a return purchase order is created following step Perform Material Inspection through Enter Inspection Results - From Warehouse app.
You want to know how the purchasing organization is determined in the return purchase order.
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SAP S/4HANA Cloud.
You can assign several purchasing organizations to a plant, if there is more than one, you can set a standard purchasing organization.
This setting is available on the organization structure, under configuration Organizational Structure.
The standard purchasing organization will be the one defined in the return purchase order:
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