The handler for the HorizontalResultsList cartridge needs to return a set of product IDs that the renderer can use to render the products on the page. This functionality already exists in the ResultsList cartridge handler, so the HorizontalResultsList cartridge is configured to use the ResultsList handler in order to avoid redundant functionality. This configuration is specified in the NucleusAssemblerFactory.handlerMapping property:

HorizontalResultsList=/atg/endeca/assembler/cartridge/handler/ResultsList

See ResultsList in this guide for details on the ResultsList cartridge handler. See Registering Cartridges with the NucleusAssemblerFactory in the ATG-Endeca Integration Guide for details on configuring a cartridge to use a specific cartridge handler. See Retrieving Renderers in that same guide for details on how the ContentItemToRendererPath component locates a renderer for a content item.