The ISO 9660 standard is a file system for optical media which allows data on CDs and DVDs to be exchanged between computers using Windows, Mac, and other operating systems. The Tip of the Night for February 19, 2016 discussed the standard in the context of .iso files which extraction software can open. It is still common for the ISO 9660 standard to be referenced in data submission standards.
Those of us who have been working with electronic media for a number of years, will recall limitations on the number of characters that can be used in the names of files saved to discs. The ISO 9660 level 1 specification restricts file names to eight characters and extensions to three characters. Levels 2 and 3 allow file name of 32 characters. Microsoft added an extension to ISO 9660, Joliet, which allows file names to be 64 characters. Directory depth is limited to 8 subfolders. There is a limit of 65,535 directories on a disc.
The Universal Disk Format (UDF) is an alternative file system for optical media. It allows for data storage of up to 2 TB, and can use file names of up to 255 characters. With UDF, it's possible for ISO 9660, Joliet and UDF formats to used on a single disc. A UDF formatted disc can hold up to 4,294,967,296 files.