A hidden index file ending in a .fts (full-text search) extension used by the Windows 95 and NT Help system. The first time you select the Find tab in a Windows help file, the system displays a dialog box asking what type of index you want to create. After you make a selection, the system creates an index and stores it in an FTS file.
You can delete an FTS file without harming your system. The help system will recreate the file next time it's needed.