The CHI files extension is used for Help Index files by Microsoft, particularly in processes where the Help Files are stored on CD-ROM. CHI files cannot be modified on the CD-ROM, but may be edited after opening and saving to a new location with the appropriate software.The CHI file extension is also used by the ChiWriter scientific word processor for document files. Created by Horstmann Software, ChiWriter has been out of production since 1995.
Another program that employs the .chi file extension is the TOMBO Note Taker program. A native application of Windows CE, it is a note management application that functions similarly as a notepad, but with added features, like multiple pane view and encryption methods for sensitive documents. This has been considered as the outgrowth development for the CHI extension from the MS-DOS environment to mobile operating system platforms