Can you please tell which pre-built integrations you are using?
Not sure if both item id and item code are always created. For example, in the Oracle Inventory, the item id is created, but not sure if item code needs to be generated.
For something like an item category, maybe code makes sense, but for something such as the item itself id is better?
If you have Item ID as the unique identifier of an Item in an application (along with Item Code) then you should use Item ID. If you don't have item ID for an application then using Item code makes sense. Your objective is to use a combination in XREF to identify the item uniquely in that application. If you have the Item in multiple Organizations then you may want to use organization ids too along with item id.