In the ASCII and Unicode , this character is encoded by the number 26 (1A hexadecimal). Standard keyboards transmit this code when the Control key and keys are pressed simultaneously (Ctrl+Z, by convention often described as ^Z).
In CP/M, 86-DOS, MS-DOS, PC DOS, DR-DOS, and their various derivatives, the SUB character was also used to indicate the end of a character stream, and thereby used to terminate user input in an interactive command line window (and as such, often used to finish console input redirection, e.g. as instigated by COPY CON: TYPEDTXT.TXT).
While no longer technically required to indicate the end of a file, many text editors and program languages still support this convention, or can be configured to insert this character at the end of a file when editing, or at least properly cope with them in text files. In such cases, it is often termed a "soft" EOF, as it does not necessarily represent the physical end of the file, but is more a marker indicating that "there is no useful data beyond this point". In reality, more data may exist beyond this character up to the actual end of the data in the file system, thus it can be used to hide file content when the file is entered at the console or opened in editors. Many file format standards (e.g. PNG or GIF) include the SUB character in their headers to perform precisely this function. Some modern text file formats (e.g. CSV-1203 CSV-1203 format specification ) still recommend a trailing EOF character to be appended as the last character in the file. However, typing does not embed an EOF character into a file in either MS-DOS or Microsoft Windows, nor do the APIs of those systems use the character to denote the actual end of a file.
Some programming languages (e.g. Visual Basic) will not read past a "soft" EOF when using the built-in text file reading primitives (INPUT, LINE INPUT etc.), and alternate methods must be adopted, e.g. opening the file in binary mode or using the File System Object to progress beyond it.
Character 26 was used to mark "End of file" even if the ASCII calls it Substitute, and has other characters for this. Number 28 which is called "File Separator" has also been used for similar purposes.
The Unicode Security Considerations report recommends this character as a safe replacement for unmappable characters during character set conversion.
In many GUIs and applications ( on Mac OS) can be used to undo the last action. In many applications earlier actions than the last one can also be undone by pressing multiple times. was one of a handful of keyboard sequences chosen by the program designers at Xerox PARC to control text editor. Presumably these particular were chosen because of their location on a standard QWERTY keyboard, since the Z (undo), control-X (cut), control-C (copy), and control-V (paste) keys are located together at the left end of the bottom row of the standard QWERTY keyboard.