Up to three character set conversions may be required for data definition language (DDL) during an export/import operation:
Export writes export files using the character set specified in the NLS_LANG environment variable for the user session. A character set conversion is performed if the value of NLS_LANG differs from the database character set.
If the export file's character set is different than the import user session character set, then Import converts the character set to its user session character set. Import can only perform this conversion for single-byte character sets. This means that for multibyte character sets, the import file's character set must be identical to the export file's character set.
A final character set conversion may be performed if the target database's character set is different from the character set used by the import user session.
To minimize data loss due to character set conversions, ensure that the export database, the export user session, the import user session, and the import database all use the same character set.