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

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

FFFE

<not a character>
* the value FFFE is guaranteed not to be a Unicode character at all
* may be used to detect byte order by contrast with FEFF which is a character
x (zero width no-break space - FEFF)

FFFF

<not a character>
* the value FFFF is guaranteed not to be a Unicode character at all