A catalogue or item number is a unique identifier assigned to a CD or other physical product. This number is used to identify and list the product in retail and distribution. Unlike the ISRC (International Standard Recording Code), UPC (Universal Product Code) or EAN (European Article Number), the catalogue number or item number does not refer to the recording itself. Instead, it is used to identify and organise the product as a whole. The catalogue number or article number is usually assigned by the company or label that publishes the product. It can consist of letters, numbers or a combination of both and is unique to each product. For example, the number may appear on the CD cover, the back of the case, or on the label of the CD itself.
This number is used to track the product at the point of sale, manage orders, track inventory, and ensure listing in distribution catalogues or online shops. It helps to uniquely identify the product and make it easier for buyers and retailers to find and order it.

It is important to note that the catalogue or item number is independent of other identification codes. Each code has a function and is used to identify and manage different aspects of the product.