A CDI file is a DiscJuggler image that allows 1GB GD-ROM data to be compressed or "downsampled" to fit onto standard 700MB CD-Rs.
For those looking to curate a Dreamcast CDI collection in 2026, several community pillars provide reliable access: Dreamcast Cdi Collection
The "backdoor" that allows these files to run is the MIL-CD (Multimedia Interactive Live CD) format. Sega originally included this for interactive music CDs, but hackers used it to bypass copy protection, allowing the console to boot unauthorized code from regular CD-Rs without a modchip. A CDI file is a DiscJuggler image that