What's the purpose of the noncharacters U+FDD0 to U+FDEF?

U+FFFE needs to be a noncharacter in order to allow the Byte Order Mark to work.

U+FFFF is described in The Unicode Standard as "useful for internal purposes as sentinels". Makes sense.

But I can't figure out, and The Unicode Standard doesn't really explain, why the set of noncharacters includes some random block within "Arabic Presentation Forms-A". What are these for? (Besides the eye of the basilisk?)

OK the question is "what are they for" and "Why are they in the middle of the Arabic Presentation Forms".

• There was a need for a block of 32 non-characters "to make additional codes available to programmers to use for internal processing purposes" http://scripts.sil.org/cms/scripts/page.php?site_id=nrsi&id=IWS-Chapter04a#4d3110c8
• It was required that it be in the Basic Multilingual Plane (BMP), i.e. 0x0000 to 0xFFFF, so that they could have single-codepoint representations in UTF-16.
• There was a block of unused codepoints in the Arabic Presentation Forms block.
• It had been agreed not to encode any more Arabic Presentation Forms, so these were never going to be used. http://www.unicode.org/mail-arch/unicode-ml/y2001-m10/0014.html

Therefore it was agreed that these codepoints, which were never going to be used otherwise, would be designated noncharacters so they could be used internally by applications/programmers.

These noncharacters are for internal use by application and should not be interchanged.

I tried to explain based on what is said in Unicode standard.

Unicode got 66 non-characters. For all 17 planes they have two each, last two code points of the plane ending with FFFE FFFF. 32 other no-characters are continuous block U+FDD0 to U+FDEF.

So total count

 17*2 + 32 = 66

Read following text from the unicode chapter 16, which says that its in some random place because of "historic reason", I'm curious but I don't think there is any ambiguity.

> For historical reasons, the range U+FDD0..U+FDEF is contained within the Arabic Presentation Forms-A block, but those noncharacters are not > "Arabic noncharacters" or "right-to-left noncharacters," and are not distinguished in any > other way from the other noncharacters, except in their code point values

U+FEFF is BOM and U+FFFE is byte-swapped version of it. But since U+FFFE is a noncharacter, when an interpreting process finds U+FFFE as the first character, it signals either that the process has encountered text that is of the incorrect byte order or that the file is not valid Unicode text, It just gives a signal, not a standard way. It can be either of the one, reverse bytes or a wrong text.

In the Unicode section 3.2 clause C2 says

> C2 A process shall not interpret a noncharacter code point as an abstract character. > > - The noncharacter code points may be used internally, such as for sentinel values or delimiters, but should not be exchanged publicly.

So as application developers you are free to use these characters as you wish. They are used as sentinel or delimter or may be some baslik characters, but they should not be interchanged.

Section 16.7 says

> In effect, noncharacters can be thought of as application-internal private-use code points. > Unlike the private-use characters discussed in Section 16.5, Private-Use Characters, which > are assigned characters and which are intended for use in open interchange, subject to > interpretation by private agreement, noncharacters are permanently reserved (unassigned) > and have no interpretation whatsoever outside of their possible application-internal private uses

Again U+FFFF is not reserved as sentinel by Unicode standard but just given the typical use case. Read in section 16.7

> U+FFFF and U+10FFFF. These two noncharacter code points have the attribute of being > associated with the largest code unit values for particular Unicode encoding forms. In > UTF-16, U+FFFF is associated with the largest 16-bit code unit value, FFFF₁₆ > U+10FFFF is associated with the largest legal UTF-32 32-bit code unit value, 10FFFF₁₆ > This attribute renders these two noncharacter code points useful for internal purposes as sentinels. For > example, they might be used to indicate the end of a list, to represent a value in an index > guaranteed to be higher than any valid character value, and so on

As mentioned here at xkcd, U+FDD0 is actually the Unicode character for the eye of a basilisk. For (obvious) reasons of personal safety however, the character is not rendered to the screen... :)