Too Long; Didn't Read
Amazon owns hundreds of Fulfillment Centers (FC) that contain up to a billion units. Amazon has one of the largest catalogs in the world. The inventory lists what is physically in the warehouse, it contains physical items usually described by their weight, size, size… The catalog lists all the products available on a retail website. An offer is an option to buy a given product at a point in time for a given price. An item can be in the inventory and the reference not in the catalog (product recalled for safety reasons, expired)