Writing about web page http://www.oracle.com/technology/pub/columns/trute_unicode.html
Oracle9i introduced length semantics, which provides the choice of whether to declare strings in terms of characters or bytes. Byte semantics is still the default, and character-length semantics can be declared at column, table, session, and for the entire database. Length semantics makes storage management more intuitive and allows a common database schema to run successfully on different databases with different character sets. This makes migrating applications and databases to Unicode much simpler.