In Unicode there are a lot of invisible, non-printable characters: regular white-space characters (e.g. U+0020 SPACE), language specific fillers (e.g. U+3164 HANGUL FILLER of the Korean Hangual alphabet), or special characters (e.g. U+2800 BRAILLE PATTERN BLANK). While all of these have a specific meaning in their natural context, they can be used in various applications that don't allow for regular whitespace characters.