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A filename extension, file name extension or file extension is a suffix to the name of a computer file (for example,
.md). The extension indicates a characteristic of the file contents or its intended use. A filename extension is typically delimited from the rest of the filename with a full stop (period), but in some systems it is separated with spaces.
Some file systems implement filename extensions as a feature of the file system itself and may limit the length and format of the extension, while others treat filename extensions as part of the filename without special distinction.
One thing you need to know about creating files with z/VM is that each file needs its own three-part identifier. The first part of the identifier is the file name. The second part is the file type. And the third part is the file mode. These three file identifiers are often abbreviated fn ft fm.