FFFE and FFFF are not unassigned in the usual sense, but guaranteed not to be a Unicode character at all. They can be used to guess a text's encoding scheme, since any text containing these is by definition not a correctly encoded Unicode text. Unicode's {{unichar|FEFF|Byte order mark}} character can be inserted at the beginning of a Unicode text to signal its endianness: a program reading such a text and encountering 0xFFFE would then know that it should switch the byte order for all the following characters.
Consider a text file containing the German word "für" in the ISO-8859-1 encoding (0x66 0xFC 0x72). This file is now opened with a text editor that assumes the input is UTF-8. The first and last byte are valid UTF-8 encodings of ASCII, but the middle byte (0xFC) is not a valid byte in UTF-8. Therefore, a text editor could replace this byte with the replacement character symbol to produce a valid string of Unicode code points. The whole string now displays like this: "f�r".
A poorly implemented text editor might save the replacement in UTF-8 form; the text file data will then look like this: 0x66 0xEF 0xBF 0xBD 0x72, which will be displayed in ISO-8859-1 as "f�r" (see mojibake). Since the replacement is the same for all errors this makes it impossible to recover the original character. A better (but harder to implement) design is to preserve 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 indicator to the user.
It has become increasingly common for software to interpret invalid UTF-8 by guessing the bytes are in another byte-based encoding such as ISO-8859-1. This allows correct display of both valid and invalid UTF-8 pasted together. If a web page uses ISO-8859-1 (or Windows-1252) but specifies the encoding as UTF-8, most web browsers used to display all non-ASCII characters as �, but newer browsers translate the erroneous bytes individually to characters in Windows-1252, so the replacement character is less frequently seen.
The following Unicode-related documents record the purpose and process of defining specific characters in the Specials block:
| Version | Final code points| Count | UTC ID | L2 ID | WG2 ID | Document | |
|---|
| 1.0.0 | U+FFFD | 1 | (to be determined) |
| 2.1 | U+FFFC | 1 | N1365 | title=Proposal Summary - Object Replacement Character|date=1996-03-18|first=Murray|last=Sargent}} |
| X3L2/96-038}} | N1354.html,}} {{nobr|N1354.doc}} | title=Resolutions from the SC2/WG2 meeting in Copenhagen, April 1996|date=1996-04-26|section=RESOLUTION M30.8}} |
| L2/97-288}} | N1603 | title=Unconfirmed Meeting Minutes, WG 2 Meeting # 33, Heraklion, Crete, Greece, 20 June - 4 July 1997|date=1997-10-24|first=V. S.|last=Umamaheswaran|section=7.3}} |
| [https://www.unicode.org/L2/L1998/98004R.pdf L2/98-004R]}} | N1681 | title=Text of ISO 10646 - AMD 18 for PDAM registration and FPDAM ballot|date=1997-12-22}} |
| [https://www.unicode.org/L2/L1998/98070.pdf L2/98-070]}} | title=Minutes of the joint UTC and L2 meeting from the meeting in Cupertino, February 25-27, 1998|first1=Joan|last1=Aliprand|first2=Arnold|last2=Winkler|section=Additional comments regarding 2.1}} |
| L2/98-318}} | N1894 | title=Revised text of 10646-1/FPDAM 18, AMENDMENT 18: Symbols and Others|date=1998-10-22}} |
| 3.0 | U+FFF9..FFFB | 3 | [https://www.unicode.org/L2/L1997/97255r.pdf L2/97-255R]}} | title=Approved Minutes - UTC #73 & L2 #170 joint meeting, Palo Alto, CA - August 4-5, 1997|date=1997-12-03|first=Joan|last=Aliprand|section=3.D Proposal for In-Line Notation (ruby)}} |
| [https://www.unicode.org/L2/L1998/98055.pdf L2/98-055]}} | title=Support for Implementing Inline and Interlinear Annotations|date=1998-02-22|first=Asmus|last=Freytag}} |
| [https://www.unicode.org/L2/L1998/98070.pdf L2/98-070]}} | title=Minutes of the joint UTC and L2 meeting from the meeting in Cupertino, February 25-27, 1998|first1=Joan|last1=Aliprand|first2=Arnold|last2=Winkler|section=3.C.5.}} |
| L2/98-099}} | N1727 | title=Support for Implementing Interlinear Annotations as used in East Asian Typography|date=1998-03-18|first=Asmus|last=Freytag}} |
| [https://www.unicode.org/L2/L1998/98158.html L2/98-158]}} | title=Draft Minutes - UTC #76 & NCITS Subgroup L2 #173 joint meeting, Tredyffrin, Pennsylvania, April 20-22, 1998|date=1998-05-26|first1=Joan|last1=Aliprand|first2=Arnold|last2=Winkler|section=Inline and Interlinear Annotations}} |
| L2/98-286}} | N1703, {{nobr>N1703ai.doc}} | title=Unconfirmed Meeting Minutes, WG 2 Meeting #34, Redmond, WA, USA; 1998-03-16--20|date=1998-07-02|first1=V. S.|last1=Umamaheswaran|first2=Mike|last2=Ksar|section=8.14}} |
| L2/98-270}} | title=Suggestion to the inline and interlinear annotation proposal|date=1998-07-29|first1=Hideki|last1=Hiura|first2=Tatsuo|last2=Kobayashi|authorlink2=Tatsuo Kobayashi}} |
| [https://www.unicode.org/L2/L1998/98281r.pdf L2/98-281R]}} | title=Unconfirmed Minutes - UTC #77 & NCITS Subgroup L2 # 174 JOINT MEETING, Redmond, WA -- July 29-31, 1998|date=1998-07-31|first=Joan|last=Aliprand|section=In-Line and Interlinear Annotation}} |
| L2/98-363}} | [https://unicode.org/wg2/docs/n1861.pdf N1861] | title=Ruby markers|date=1998-09-01|first=T. K.|last=Sato}} |
| L2/98-372}} | [https://unicode.org/wg2/docs/n1884.pdf N1884R2.pdf],}} {{nobr|[https://unicode.org/wg2/docs/n1884.doc N1884R2.doc]}} | title=Additional Characters for the UCS|date=1998-09-22|first1=Ken|last1=Whistler|display-authors=etal}} |
| L2/98-416}} | [https://unicode.org/wg2/docs/n1882.zip N1882.zip]}} | title=Support for Implementing Interlinear Annotations|date=1998-09-23}} |
| [https://www.unicode.org/L2/L1998/98312.pdf L2/98-312]}} | title=Resolutions from SC2/WG2 meeting in London with comments from Ken Whistler|date=1998-09-29|first=Ken|last=Whistler|section=N1882 Interlinear annotation characters}} |
| [https://www.unicode.org/L2/L1998/02n3210.pdf L2/98-329]}} | N1920 | title=Combined PDAM registration and consideration ballot on WD for ISO/IEC 10646-1/Amd. 30, AMENDMENT 30: Additional Latin and other characters|date=1998-10-28}} |
| [https://www.unicode.org/L2/L1998/98421.htm L2/98-421R]}} | title=Notes concerning the PDAM 30 interlinear annotation characters|date=1998-12-04|first1=Michel|last1=Suignard|first2=Hideki|last2=Hiura}} |
| [https://www.unicode.org/L2/L1998/98419.pdf L2/98-419]}} | title=Approved Minutes -- UTC #78 & NCITS Subgroup L2 # 175 Joint Meeting, San Jose, CA -- December 1-4, 1998|date=1999-02-05|first=Joan|last=Aliprand|section=Interlinear Annotation Characters}} |
| [https://www.unicode.org/L2/L1999-UTC/u1999-021.txt UTC/1999-021]}} | title=W3C XML CG statement on annotation characters|date=1999-06-08|first1=Martin|last1=Duerst|first2=Jon|last2=Bosak|authorlink2=Jon Bosak}} |
| [https://www.unicode.org/L2/L2001/01301-deprecation.txt L2/01-301]}} | title=Analysis of Character Deprecation in the Unicode Standard|date=2001-08-01|first=Ken|last=Whistler|section=E. Indicated as "strongly discouraged" for plain text interchange}} |
1. ^{{cite web|url=https://www.unicode.org|title=Unicode character database|work=The Unicode Standard|accessdate=2016-07-09}}
2. ^{{cite web|url=https://www.unicode.org/versions/enumeratedversions.html|title=Enumerated Versions of The Unicode Standard|work=The Unicode Standard|accessdate=2016-07-09}}
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