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

Question

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 sentine

Answer 1

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.