I realise that people like to short-hand things but I don’t actually see how the item code in Manager has a use. The drop down box to select an item shows the full item description anyway, and the item code looks kinda unprofessional printed right next to the full item description on a quote.
What is the idea behind the item code?
From my understanding, item code is a general term used to indicate Stock Keeping Unit (SKU) which is commonly used for ecommerce sellers to keep their stock availability in check automatically. Correct me if I’m wrong. Cheers 
This is explained in the guides at https://www.manager.io/guides/7551 i.e.
Item code can be anything that helps find, sort, or categorize the item. This could be a stock-keeping unit (SKU) code, a model number, or a simple memory aid. It must be unique and should be short. Codes will appear on forms if they are used. Serial numbers are not acceptable item codes, because every unit of inventory would be treated as a separate product.