G64
A disk image that stores the 1541's raw GCR bitstream instead of finished sectors — so it can reproduce copy protection a D64 smooths away.
Where a D64 assumes a standard-formatted diskette and stores the decoded sectors, a G64 keeps the track contents as they physically lie on the surface: raw GCR, sync marks and all. The format was devised for the VICE emulator and became the way to preserve disks that deliberately break the standard.
That matters for anything that protected itself by breaking the rules: extra half-tracks, odd sector lengths, intentionally corrupt sync, or weak bits that read differently every pass. A D64 can express none of it; a G64 can. The price is larger files and having to read the image at the GCR level with a nibbler rather than through DOS.