CJK Compatibility Ideographs is a Unicode block created to contain mostly Han characters that were encoded in multiple locations in other established character encodings, in addition to their CJK Unified Ideographs assignments, in order to retain round-trip compatibility between Unicode and those encodings.
However, it also contains 12 unified ideographs sourced from Japanese character sets from IBM.
[4][5] These sequences specify the desired glyph variant for a given Unicode character.
Sources for the original collection of CJK Compatibility Ideographs include: In ensuing versions of the standard, more characters have been added to the block from: IBM Japanese double-byte EBCDIC includes several kanji which do not exist in, or do not round-trip from, JIS X 0208.
These were included as gaiji in extensions to Shift JIS and EUC-JP from IBM (e.g. code page 942), NEC, the Open Software Foundation, and Microsoft (e.g. Windows code page 932).