Order upload can use any of the following as the "Item Number" in the file. However, not all of these are guaranteed to be unique. When there is a duplicate ManufactureItem number, for example, Opti just picks the first one and adds it to the Cart. Opti does not say "Wait, this is a duplicate. Hey user, what do you want me to do with this?" The result is the wrong product can getting added to Cart.
* CPN.Name
* ErpNumber
* ManufacturerItem
* Sku
* UPCCode
* Unspsc