When publishing lists other than the standard Product Catalog list as catalog, map the Crawled Property linked to the Site Column representing the unique identifier of each catalog item to the UsageAnalyticsId Managed Property.
When publishing content as catalog in SharePoint 2013 you have to have a Site Column in your catalog that will uniquely identify each catalog item. For products this could be SKU number, for books ISBN and for web pages slug (search-optimized page name). This will allow you to properly retrieve your catalog items when published.
One of the great capabilities that SharePoint 2013 Search offers is displaying content recommendations based on the usage pattern. By default recommendations work only for products stored in the standard Products Catalog List. This has to do with the fact that SharePoint uses the UsageAnalyticsId Managed Property which is mapped to the ows_ProductCatalogItemNumber Crawled Property. If you want recommendations to work with your custom content as well, you should map the UsageAnalyticsId Managed Property to the Crawled Property corresponding to the unique identifier Site Column for your content catalog.