See also the corresponding code chart.

FFF0 Specials FFFF

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>