Crawl or Arc Folder to Re-index dropdown allows one to select a previous Yioop crawl or an archive to do another crawl of. Possible archives that can be index include Arc files, Warc Files, Email, Database dump, Open Directory RDF dumps, Media Wiki dumps etc. Re-crawling an old crawl might be useful if you would like to do further processing of the records in the index. Besides containing previous crawls, the dropdown list is populated by looking at the WORK_DIRECTORY/archives folder for sub-folders containing an arc_description.ini file.