Specials

Interlinear annotation

Used internally for Japanese Ruby (furigana), etc.
FFF9  Interlinear Annotation Anchor
  marks start of annotated text
FFFA  Interlinear Annotation Separator
  marks start of annotating character(s)
FFFB  Interlinear Annotation Terminator
  marks end of annotation block

Replacement characters

FFFC  Object Replacement Character
  used as placeholder in text for an otherwise unspecified object
FFFD � Replacement Character
  used to replace an incoming character whose value is unknown or unrepresentable in Unicode
  compare the use of 001A  as a control character to indicate the substitute function
  2BD1 ⯑ uncertainty sign
  1FBC4 🯄 negative squared question mark

Noncharacters

These codes are intended for process-internal uses.
FFFE █ <Not A Character>
  may be used to detect byte order by contrast with FEFF 
  FEFF  zero width no-break space
FFFF █ <Not A Character>