When internationalizing applications for Unicode, developers should use the POSIX or X Window model. These models define two sets of interfaces--multibyte and wide character--without specifying the encoding methods.
Standard multibyte codesets contain characters of varying widths; from one to several bytes. Characters are represented in minimal storage space, with the fewest number of bytes possible. Because multibyte codesets contain characters of varying widths, they are not conveniently processed by standard functions.
The Unicode codeset provides the necessary format for both multibyte and wide-character representation. In the Solaris operating environment Unicode locales, multibyte interfaces use UTF-8 character set representation and wide-character interfaces use UCS-4 representation.