AnswerWhen running Device Usage Summary reports in the GFI EndPointSecurity ReportPack, there are two types of files listed, file system files and non file system files. File system files are files that are viewable in Windows Explorer. Files identified as being non-file system files are files located within NTFS' Alternate Data Stream. Alternate data streams were introduced with NTFS in the Windows NT operating system. They were created to provide compatibility with HFS-based file systems used by Macintosh computers. Alternate data streams are not widely used today, but they have been used in security exploits for some time, such as in viruses and other forms of malware. They are used because these streams of data are not seen using normal viewing methods, such as a command prompt or Windows Explorer.