It has been occasionally used by phonologists to represent a hypothetical phoneme in English, which includes both [h] and [ŋ] as its allophones, to illustrate the limited usefulness of minimal pairs to distinguish phonemes. Normally /h/ and /ŋ/ are considered separate phonemes in English, even though a minimal pair for them cannot be constructed, due to their complementary distribution.[1]
basically, in English you only get /h/ at the start of syllables and only get /ŋ/ (the "ng" sound) at the end of syllables. randomly, this is also the case in Korean, where the ㅇ letter has dual uses: it represents /ŋ/ at the end of a syllable and no consonant at the start of a syllable. (sadly, h uses the slightly different character ᄒ)
