You can freely assign 5 reference numbers per order and per product in Keyline.
Whether orderer, cost center, order number, etc. of the designation are no limits set.
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Keyline also indexes the references, so that you can quickly and directly access the respective order or product by entering it in the search field. 

Reference numbers at order level appear strikingly on the documents directly below the offer number. Global for the complete order.
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You can also create the reference numbers for the products, then these appear in the description text of the respective product with the offer, the order confirmation and the invoice.