Linguists use a variety of symbols to represent not just single sounds, but certain particular classes of sounds. They are usually capital letters. This article lists those "cover symbols".
Vowels
Capitalised vowels are commonly used in discussions of languages with vowel harmony. They often indicate different harmonic variants of an underlying archiphonemic vowel.
Some vowel symbols in the International Phonetic Alphabet, such as ⟨ə, ø, y⟩ are also sometimes used as cover symbols.
References
Cover symbols used in linguistics Wikipedia(Text) CC BY-SA