A: A Unicode transformation format (UTF) is an algorithmic mapping from every Unicode code point (except surrogate code points) to a unique byte sequence. The ISO/IEC 10646 standard uses the term “UCS transformation format” for UTF; the two terms are merely synonyms for the same concept.
Each UTF is reversible, thus every UTF supports lossless round tripping: mapping from any Unicode coded character sequence S to a sequence of bytes and back will produce S again. To ensure round tripping, a UTF mapping must map all code points (except surrogate code points) to unique byte sequences. This includes reserved (unassigned) code points and the 66 noncharacters (including U+FFFE and U+FFFF).
The SCSU compression method, even though it is reversible, is not a UTF because the same string can map to very many different byte sequences, depending on the particular SCSU compressor.