To test if a file has changed over time. A checksum is a type of metadata and an important property of a data object to allow verifying identity and integrity. Also called a hash, a checksum is a randomly generated piece of data that is used to verify the fixity or stability of a digital object. It is most commonly used to detect whether some representation of digital object has changed over time. This is associated with PIDs but can be found and tested independently of PID systems.