What are two functions of a specific filename extension?
Filename extensions are a form of identifier for programs and files; a text file might have .txt or .doc after its proper name, denoting either a plaintext file or a MSWord document with higher-level formatting. The extension allows an operating system to classify and sort files according to their function; the OS also associates other programs with the files based on the extension. For example, on a Windows PC the file example.txt would associate with Notepad, a simple text editor, while example.doc would associate with MSWord or similar, a more powerful editor. Example.rtf might associate with several programs because the .rtf denotes Rich Text Format, which is more universal. By identifying the file according to a generic list, the OS can store data about the file as well as remember it for searching. Some filename extensions have more than one function; a .exe file can open as a runtime application, link as a shortcut to a different application, or operate as a batch file to run one or more separate programs (a batch file is often .bat, but sometimes is listed as .exe to avoid confusion).