The results are organized in one list of results. Metasearch engines have some limitations as comprehensive search tools, since they usually do not provide advanced search features and often do not produce as many hits as a search on a single search engine. Invisible Web Search Engines provide access to resources inaccessible to general search engines. These specialized search engines offer access to documents hidden in databases, provide highly relevant searches limited to specific subject content, or search for specific types of resources, including graphics, audio, and video files. Subject directories provide subject access to sites which have been reviewed, selected and sometimes rated for quality.
Many subject directories are both searchable and provide a browsable directory of subject headings.