A file with a "Zone.Identifier" extension is an alernate data stream (ADS) file that contains information about another file. It describes the security zone associated with the file, which may the Internet, local intranet, trusted site, or restricted site.

Zone identifier files are generated automatically by Internet Explorer and other programs when files are downloaded to a Windows computer. These files are used by Windows manage security settings for specific files. They are typically hidden and are not meant to be opened directly.