Description: The PRONOM Persistent Unique Identifier (PUID) is an extensible scheme for providing persistent, unique and unambiguous identifiers for records in the PRONOM registry. Such identifiers are fundamental to the exchange and management of digital objects, by allowing human or automated user agents to unambiguously identify, and share that identification of, the representation information required to support access to an object. A PUID is assigned on entry to PRONOM, and every identifier is unique within the namespace, and is persistently and unambiguously bound to a single registry entry. The PUID type element identifies the type of registry entry; for a file format it is fmt or x-fmt; for a character encoding it is chr or x-chr; for a compression encoding it is cmp or x-cmp; for a software package it is sfw or x-sfw. The integer value that follows is the identifier element and denotes an instance of that type within PRONOM. The x-puid is a legacy form of the PRONOM Persistent Unique Identifier (PUID) originally intended as a temporary identifier to assign to records until a judgment could be made about the validity of the entry; if the entry was deemed to be a valid entry then a standard PUID was meant to replace the X-PUID on an entry. Many collections have been characterised using X-PUIDs and for this reason existing X-PUIDs are still maintained but will not be assigned to any new entries.