The Customer can enter the External Item Description information belong to vendors' product records. Also, those records put on the purchase order lines as reference and text fields. But, If the vendor has multiple external item records with different units, there is no place to write this. Quantity, Package, Pallete etc. When the customer wants to send a purchase order document to the vendor with the right units' product reference code, dynamics 365 FO does not allow this. Because of unique index constraints. Index constraints may be changed or new external item description table designed for units.