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.
It is mainly used for DVD-Video discs, but also by software such as DirectCD for Windows by Roxio, InCD by Ahead, Write UDF by Software Architects.
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).
UDF is fully compliant with the ISO 13346 standard.
UDF has been released in multiple revisions:
Revision 2.50. Added the Metadata Partition facilitating metadata clustering and optional duplication of file system information.
Revision 2.01. Added support for Real Time Files.
Revision 2.00. Added support for Stream Files, Access Control lists, and Power Calibration.
Revision 1.50. Added support for (virtual) rewritability on CD-R and CD-RW media by intoducing the VAT structure, as used in file systems such as the product DirectCD by Roxio.
Revision 1.02. This format is used by DVD-Video disks.
Modern operating systems understand 1.02 and 2.01 revisions of UDF (well, Mac OS 9 and Mac OS X (10.3) do only support 1.02 without the help of 3rd party software, meaning that Mac OS can read DVD-Video Disks but not CDs created by Roxio's DirectCD and similar tools using UDF 1.50 or later)
A contradictory rumour says Mac OS X 10.3.2 mounts and reads (but does not write) UDF 1.50 on DVD-RAM discs erased via such commands as Linux `mkudffs --udfrev=0x0150 /dev/scd$n`.