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UTF-32 (32-bit Unicode Transformation Format), sometimes called UCS-4, is a fixed-length encoding used to encode Unicode code points that uses exactly 32 bits (four bytes) per code point (but a number of leading bits must be zero as there are far fewer than 2 32 Unicode code points, needing actually only 21 bits). [1]
UTF-8, UTF-16, UTF-32 and UTF-EBCDIC have these important properties but UTF-7 and GB 18030 do not. Fixed-size characters can be helpful, but even if there is a fixed byte count per code point (as in UTF-32), there is not a fixed byte count per displayed character due to combining characters. Considering these incompatibilities and other quirks ...
The BOM for little-endian UTF-32 is the same pattern as a little-endian UTF-16 BOM followed by a UTF-16 NUL character, an unusual example of the BOM being the same pattern in two different encodings. Programmers using the BOM to identify the encoding will have to decide whether UTF-32 or UTF-16 with a NUL first character is more likely.
For UTF-8, the BOM is optional, while it is a must for the UTF-16 and the UTF-32 encodings. (Note: UTF-16 and UTF-32 without the BOM are formally known under different names, they are different encodings, and thus needs some form of encoding declaration – see UTF-16BE, UTF-16LE, UTF-32LE and UTF-32BE.) The use of the BOM character (U+FEFF ...
Some programming languages, such as Seed7, use UTF-32 as an internal representation for strings and characters. Recent versions of the Python programming language (beginning with 2.2) may also be configured to use UTF-32 as the representation for Unicode strings, effectively disseminating such encoding in high-level coded software.
Even Microsoft will recognize UTF-16 without the BOM, and I kind of doubt much software recognizes UTF-32 using it (as it will look like UTF-16 starting with a BOM and a NUL). Therefore there are two reasons: identify byte order in UTF-16 and UTF-32, and as a marker to distinguish UTF-8 from legacy 8-bit encodings.
It is increasingly common for multilingual websites and websites in non-Western languages to use UTF-8, which allows use of the same encoding for all languages. UTF-16 or UTF-32, which can be used for all languages as well, are less widely used because they can be harder to handle in programming languages that assume a byte-oriented ASCII ...
UTF-7, an obsolete encoding, had an advantage over Unicode encodings, on obsolete non-8bit-clean networks, in that it does not require a transfer encoding to fit within the seven-bit limits of legacy Internet mail servers. On the other hand, UTF-16 must be transfer encoded to fit SMTP data format.