SSD Over-Provisioning
Flash capacity kept outside the host-addressable space so an SSD controller can perform garbage collection, wear leveling, replacement and performance management.
The amount of flash physically present in an SSD does not have to equal the capacity exposed to the operating system. A controller can reserve part of the medium for garbage collection, replacement blocks and wear management; that reserve is commonly called over-provisioning.
More spare space gives the controller additional freedom under sustained writes. From a sanitization perspective, it also illustrates why deleting host-visible files is not equivalent to proving that every physical location is cleared.
The Flash Translation Layer explains the mapping, while SSD and NVMe Media Sanitization and Cryptographic Erase examines the sanitization boundary.