On Canon cameras, a normal format usually only rebuilds the card’s file-system structures (such as the FAT/file management area), so most image data remains on the card until it is overwritten later.

Canon’s “low-level format” in this context is not the old hard-drive meaning. Based on user manual notes and user testing, it means the camera overwrites the card’s data area, effectively clearing the whole partition rather than just resetting directory/file-allocation information.

So in practical terms:

• normal format: quick reset of file-system metadata
• low-level format: writes across the card to clear existing data

This is why low-level format is more thorough if you want to fully erase prior images or refresh the card for use in-camera. Canon manuals indicate this behavior for supported bodies, especially with SD cards. The exact implementation can vary by card type, but the key difference is that normal format mainly changes file management information, while low-level format overwrites the card contents.