For each item, code assigned might differ from the code your supplier has given to that same item. ERPNext allows you to track Supplier's Item Code in the item master. Also you can fetch Supplier's Item Code in your purchase transactions, so that they can easily recognize item referring to their Item Code.
Each purchase transaction has field in the Item table where Supplier's Item Code is fetched. This field is hidden in form as well as in the Standard print format. You can make it visible by changing property for this field from [Customize Form.]({{docs_base_url}}/user/manual/en/customize-erpnext/customize-form.html)
Supplier Item Code will only be fetched in the purchase transaction, if both Supplier and Item Code selected in purchase transaction is mapped with value mentioned in the Item master.