Forum smart search
On this page, you can see the Smart search dialog with results web part configured to search within a specified forum index. When a search expression is submitted, the web part also displays the results of the search.
The web part belongs to the Smart search module. Full documentation of the module can be found in Kentico CMS Developer's Guide -> Modules -> Smart search. Full documentation of the Forums module can be found in Kentico CMS Developer's Guide -> Modules -> Forums.