This validation checks if a string contains characters whose width is not right. Alphabets, Japanese Katakana and Korean Jamo characters exist both in half width and full width forms. Normally you should not only use full width alphabets or half width katakana or jamo characters.
When translating European languages on Japanese computer it sometimes happens that translator by mistake uses full with alphabet instead of half width characters.
|ファイル||File||This English translation uses half-width alphabets that is correct.|
|ファイル||Ｆｉｌｅ||This English translation uses full-width alphabets that is incorrect. Even it looks just all right it might be that end users computer can not render it correctly.|