Specials (Unicode block)

Specials is a short Unicode block of characters put at the very end of the Basic Multilingual Plane, at U+FFF0–FFFF. Of these 16 code points, five have been used since Unicode 3.0:
FFFE and FFFF are not unused in the usual way, but guaranteed not to be Unicode characters at all. They can be used to guess a text's scheme, since any text using these is not a correctly encoded Unicode text. Unicode's character can be put at the start of a Unicode text to signal its endianness: a program reading such a text and finding 0xFFFE would then know that it should switch the byte order for all the following characters.
Its block name in Unicode 1.0 was Special.
Replacement character.
The replacement character (�) (often displayed as a black rhombus with a white question mark) is a symbol found in the Unicode standard at code point U+FFFD in the "Specials" table. It is used for problems when something is unable to render a stream of data to a correct symbol. It is usually seen when the data is not valid and does not match any character:
Consider a text file containing the German word (meaning 'for') in the ISO-8859-1 encoding (codice_1). This file is now opened with a text editor that thinks that the input is UTF-8. The first and last byte are valid UTF-8 encodings of ASCII, but the middle byte (codice_2) is not a valid byte in UTF-8. So, a text editor could replace this byte with the replacement character symbol to make a valid string of Unicode code points. The whole string now looks like this: "f�r".
A poorly made text editor might save the replacement character in UTF-8, and the text file data will then look like this: codice_3, which will be displayed in ISO-8859-1 as "fï¿½r" (this is called mojibake). Since the replacement is the same for all errors this makes it impossible to get the first character. A better (but harder to make) way is to keep the original bytes, including the error, and only convert to the replacement when "displaying" the text. This will allow the text editor to save the original byte sequence, while still showing the error to the user.
At one time the replacement character was used a lot when there was no glyph that can be used in a font for that character. However, most new text rendering systems instead use a font's character, which most times is an empty box (or "?" or "X" in a box), sometimes called a "tofu" (this browser displays 􏿾). There is no Unicode point for this symbol.
So, the replacement character is now only seen for encoding errors, such as invalid UTF-8. Some software tries to hide this by translating the bytes of invalid UTF-8 to matching characters in Windows-1252 (since that is the most likely source of these errors), so that the replacement character is never seen.
History.
The following Unicode-related documents record the purpose and process of defining specific characters in the Specials block: