Substitute

The substitute symbol was used in computer terminals, printers, text processing systems, and telecommunications equipment. Its main purpose was to indicate the position in the data stream where another character or sequence of characters should be inserted.

It tended to replace invalid, damaged, or missing characters in the data stream. For example, when transmitting text between different systems, you can replace some characters that cannot be correctly interpreted. Here the SUB symbol comes in handy. It indicates the position where an appropriate character should be placed.

As for modern computer systems and applications, the U+001A symbol is rarely used there. The main reason is because other mechanisms and protocols are applied to process and replace incorrect characters.

Like many other control symbols, it has no visible representation and doesn't occupy a lot of space on screen or in typed text. However, there is a separate symbol in Control Pictures  2400–243F representing the graphical image of the Substitute symbol as the abbreviation SUB (Substitute) — ␚ .

The symbol “Substitute” is included in the “C0 controls” subblock of the “Basic Latin” block and was approved as part of Unicode version 1.1 in 1993.