How is Item Cross Reference numbers in the Item Relationships referenced to the Purchase Order?

Summary:

Would like to understand how Item cross reference values are related to Purchase Orders and if possible directly capture the value on the PO Line.

Content (required):

Client is capturing UPC and other item specific codes as cross reference values and would like to capture these value directly on the Purchase Order for historical reference if the code value is changed. How is the Cross Reference numbers in the Item Relationships referenced to the Purchase Order? Is there a direct copy of the value on one of the PO tables or is it just a reference to the value on the associated item number record?