On this tab, you can define which documents will be included in the currently edited index.
Document indexes can be defined by Alias path and Document type. You can see the content definitions in the list, while the Type column shows if the defined content is Allowed (included) or Excluded.
Excluded content is removed from the allowed content. So for example if you allow /% and exclude /Special-pages/% at the same time, it means that all pages on the site will be indexed, except for pages found under the /Special-pages node.
Using the Add allowed content and Add excluded content icons, you can access the dialog for defining new allowed/excluded content.
More resources can be found at:
|•||Developer's Guide -> Modules -> Smart search -> Managing indexes -> Defining document index content|