replacement

Specials is a short Unicode block allocated at the very end of the Basic Multilingual Plane, at U+FFF0–FFFF. Of these 16 code points, five are assigned as of Unicode 12.0:

U+FFF9 INTERLINEAR ANNOTATION ANCHOR, marks start of annotated text
U+FFFA INTERLINEAR ANNOTATION SEPARATOR, marks start of annotating character(s)
U+FFFB INTERLINEAR ANNOTATION TERMINATOR, marks end of annotation block
U+FFFC  OBJECT REPLACEMENT CHARACTER, placeholder in the text for another unspecified object, for example in a compound document.
U+FFFD � REPLACEMENT CHARACTER used to replace an unknown, unrecognized or unrepresentable character
U+FFFE <noncharacter-FFFE> not a character.
U+FFFF <noncharacter-FFFF> not a character.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 U+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.

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