With Related Ads feature our theme show other related listings on listing page. Many options you can configure for this feature. To configure related ads feature visit: Shopclass Settings=>Related Ads. It will direct you to this Related ads configuration screen.
As you can see their are many settings you can change like Number of related ads, Show Only Premium Ads, With Pictures Only, With Same Category, With same country, With Same Region and option to choose layout from two available layout.
If you have enabled Sphinx Search Feature in Shopclass theme than Related ads feature will use Sphinx Search Engine to show more better related ads.
Note: While sphinx is enabled not all filters will work.( Not Valid for version >= 1.2.0)