Optional purchase order number assigned by the merchant.
In this field the merchant may assign an optional Purchase Identifier. This field must contain digits, and will be automatically left-justified, space-filled to 25 spaces. For American Express transactions, only use the first 9 spaces.
This field is applicable to E-Commerce, Hotel, and Auto Rental transactions.
Failure to set a PurchaseIdentifier for the e-commerce IndustryType may result in a CPS downgrade when the transaction is settled. This can result in higher interchange rates.