What is a UPC code?

Short for Universal Product Code: (In USA) a unique 12-digit number assigned to retail merchandise that identifies both the album and the artist that sells the product.

Once the UPC code is registered, it tracks and reports sales. Here's a link to Nielson Soundscan - http://titlereg.soundscan.com/soundscantitlereg/

Not to be confused with an ISBN number: The ISBN identifies the title or other book-like product (such as an audiobook) to which it is assigned, but also the publisher to be contacted for ordering purposes