The Universal Disk Format (UDF) is used for so called "packet written" writable CD-ROMs that are not simply copies of ISO 9660 CDROMs. Packet writing allows CDs to be partially written using variable or fixed length records in multiple sessions. Variable length packets written CD-Rs and CD-RWs can (usually) be closed to ISO 9660 format just by writing a table of contents on the CD. Fixed length packet formatting can not be closed to ISO 9660 format, but can be randomly written and overwritten. Fixed length formatting cuts CD capacity by about 20% relative to ISO 9660 or variable length formatting.
Conventional operating systems only provide support for ISO 9660 format unless special drivers are loaded. Thus UDF formatted information can not be used or listened to without closing the file to ISO 9660 format. Once closed, the CDROM can not be reopened.
More generally, UDF is a format specification of a file system for storing files on recordable media, mainly media with limited rewriting conditions, such as:
UDF is practically the successor of ISO 9660, supporting larger files, larger disk and more information about individual files and folders (e.g., it includes support for special file properties, such as Apple's File Types, Resource Forks, and other OS-specific data).