Understanding the difference between item IDs and Item numbers can get confusing to users, we have written this article to clarify the difference between the two.
Let’s start with item number (SKU).
Item numbers are your internal item number to identify each product.
This will be used for inventory control and as the base number from which Variants build their item numbers. A synonym for Item Number is "SKU", which stands for "Stock Keeping Unit."
Item numbers can be assigned in the product editor for individual items in the general settings tab.
To assign the item number, navigate to Catalog > Products
Next select an item you wish to add an item number to and click the edit icon of that product.
Once inside the product editor, in the general tab, you can set the Item Number (SKU).
Once you have added the number, click on Save.
Now, let’s take a look at item ID’s.
Item ID’s are numbers that the system assigns to newly created products. These numbers are used by the database to keep track of each individual product.
This means that even of you change the item number (SKU), the item ID will remain the same.
This image shows how to identify what each number is in the product list: 
Unlike item numbers, item ID’s need to remain unchanged to avoid issues with the item in the database. The only way item ID’s can be edited is through a product import.
There will be some instance where you will need to know the item IDs of products, such as setting up particular types of discount methods.