FFFE and FFFF are not unused in the usual way, but guaranteed not to be Unicode characters at all. They can be used to guess a text's scheme, since any text using these is not a correctly encoded Unicode text. Unicode's character can be put at the start of a Unicode text to signal its endianness: a program reading such a text and finding 0xFFFE would then know that it should switch the byte order for all the following characters.