Overview:
Adding an Item Alias is relevant to Purchase Orders in the event that there is an alternate name used by the vendor.
How-To:
For you to create an Item Alias, you would want to go to Management > Inventory Classification > Master Item and either create a new item or search for an item you wish to update. In the "Extended Information" section, you will then put in the Item Alias (ideally, the ID the vendor uses).
Item Alias on Master Item page
Next Step:
From there, you will be able to create a purchase order, add items, and the Item Alias field will populate.
Item Alias when creating a Purchase Order
Item Alias visible on Purchase Order