What are UCS implementation levels?
Not all systems are expected to support all the advanced mechanisms of UCS such as combining characters. Therefore, ISO 10646 specifies the following three implementation levels: Level 1 Combining characters and Hangul Jamo characters are not supported. [Hangul Jamo are an alternative representation of precomposed modern Hangul syllables as a sequence of consonants and vowels. They are required to fully support the Korean script including Middle Korean.] Level 2 Like level 1, however in some scripts, a fixed list of combining characters is now allowed (e.g., for Hebrew, Arabic, Devanagari, Bengali, Gurmukhi, Gujarati, Oriya, Tamil, Telugo, Kannada, Malayalam, Thai and Lao). These scripts cannot be represented adequately in UCS without support for at least certain combining characters. Level 3 All UCS characters are supported, such that for example mathematicians can place a tilde or an arrow (or both) on any arbitrary character.
