Hangul Jamo Extended-A is a Unicode block containing choseong (initial consonant) forms of archaic Hangul consonant clusters. They can be used to dynamically compose syllables that are not available as precomposed Hangul syllables in Unicode; specifically, syllables that are not used in standard modern Korean.
The following Unicode-related documents record the purpose and process of defining specific characters in the Hangul Jamo Extended-A block: